They were named Suspect 1 and Suspect 2, and Americans who recognised them were advised to tread carefully. In fingering them as the likely perpetrators of the Boston bombing, the FBI warned they are likely to be armed and "extremely dangerous".
Police had released photos of the marathon bombing suspects and asked for the public's help finding them. Suspect 1 was pictured wearing a black baseball cap. Suspect 2 wore a white hat.
After an extraordinary night which began with a campus police officer being shot dead at the Massachusetts Institute of Technology late Thursday, one of the two suspects in the Boston Marathon bombing was dead and a massive manhunt was underway for the other.
The killing of the MIT police officer had been followed by a shootout between two young men, armed with guns and explosives, and dozens of police in Watertown, Massachusetts. Afterwards, Boston Police Commissioner Ed David said that the so-called Black Hat suspect was dead. White Hat, was "still at large, armed and dangerous."
As the FBI tried to confirm whether the individuals involved in the violence overnight were the suspects depicted in images the agency released earlier, many Americans may have been wondering if some of the country's institutions were fracturing before their eyes.
The rapid advances in the investigation showed that at least some authorities had not lost their bottle in the aftermath of the finishing-line carnage in Monday's Boston Marathon.
The FBI claims to have, but did not release, footage of one of the men positioning a black backpack on the ground just minutes before the twin explosions. Another unreleased clip was described as a clincher because it reveals nonchalant behaviour of the pair immediately after the explosions - while others fled in panic, they hung back, observing the bloody chaos before leaving the scene casually. Also, a backpack carried by one of the men was thought to be a match for the shredded remnants of a bag the FBI believes was used to carry one of the bombs.
Hours before the release of the pictures, President Barack Obama addressed the suspects personally as he spoke during an interfaith memorial service at Boston's Cathedral of the Holy Cross. Warning that they had "picked the wrong city", he told them: "Yes, we will find you. And yes, you will face justice. We will hold you accountable."
Security jitters gripped the country through the week - in New York, a sharp rise in reports of suspicious packages; in Oklahoma City, with memories of 1995's massive bombing rekindled, a bomb scare; elsewhere, tightened security. Americans needed answers, but to one question in particular: who was behind the plot that shattered Monday's "perfect day" fun at the marathon?
Almost 30,000 runners from around the world were having a mid-afternoon, metaphoric blast. Then two literal blasts, 15 seconds apart, transmuted the finishing-line cocktail of adrenalin, exhilaration and "I-did-it" self-awareness into one of those Afghan or Iraqi charnel house montages.
Three people were dead and more than 170 were badly injured - some collapsed on the blood-soaked pavements, limbs sheared off by the nails and ball-bearings packed into what appear to have been crude pressure-cooker bombs, the lid from one of which was found on the roof of a nearby building two days later.
The initial response was a heart-warming mix of courage and humanity - spectators who ran to the blasts, not away, to help the wounded; participants who kept running, over the finishing line and to the nearest hospitals to give blood; and an outpouring of sympathy from throughout the country that was coupled with promises, from the President down, that justice would be done.
Then began something of an unravelling.
It made sense that beyond an initial mix of shock and sympathy, the response of Americans would be qualified by what they would be told of the perpetrator or perpetrators.
A jihadist plot would bring Americans together, for another circling of the national wagons around "our values, our freedoms". But should it turn out that this attack was home-grown; there likely would be an ugly "them-and-us" domestic standoff - and inevitable political point-scoring and name-calling, depending on any alignment of their beliefs with the platform of either side of politics.
Suspect 1 and Suspect 2 appeared to be in their 20s. But in releasing their photo images, the FBI made no mention of their likely ethnic or national background.
In his first response, on Monday afternoon, Mr Obama seemed to take political correctness to an extreme - he refused to utter the words "terror", "terrorism" or "attack". Boston streets were awash with blood, so clearly this was an act of terror - but in the modern lexicon, "terrorism" in the US means Muslim; it connotes Middle East. Rupert Murdoch's New York Post and Fox News were the first to show signs of a malaise that would afflict more media outlets as the week progressed, greatly embarrassing the likes of CNN and the Associated Press. Apart from having a death toll four times higher than the actual figure, the Post breathlessly revealed that the police had a suspect - an injured Saudi national who was under police guard at a Boston hospital.
The Post attributed some of its information to Fox, where various on-air personalities talked it up - "There must be enough evidence to keep him there"; "They must be learning information which is of a suspicious nature"; "If he was clearly innocent, would they be able to search his house?"; "What's the story on his ability to lawyer up?"
But all this was met with denial by the authorities. Indeed, there was a 20-year-old Saudi, but like the 170-odd others injured, he was a witness who, like so many others who were near the finishing line, ran when he heard the explosions. "The range of suspects and motives remains wide open," the FBI's Boston chief, Richard DesLauriers, told reporters.
The absence of information of the identity of those behind the attack also drove anxiety among American Muslims - fear of a backlash reportedly prompted the leadership at Boston's biggest mosque to request a police guard.
Impressive investigative work turned up parts of one or two domestic pressure cookers that were the bomb casings, and a sense of how they worked - nails and ball bearings packed with gunpowder, a circuit board, a kitchen timer for a trigger and a battery of the kind used in some remote-control toys.
The plan was to track them through their manufacturers to where they were sold in the hope that the retailers might have information on who bought them.
These bombs were as crude as they were cheap - probably costing less than $100 each. And despite the impact, they packed a small punch - yes, appalling injuries to those who were close by, but no cratered pavements or structural damage to buildings.
Compared with the impact of such attacks in Kabul and Baghdad, where more than 40 died and 250 were injured in a series of bombs on the same day, the Boston bomb would have been rated a jihadist disappointment.
On Thursday, CNN's John King told the world that an individual that authorities had identified as a suspect from a department store CCTV security tape was "a dark-skinned male". He went on in what seemed to be a code: "The official [who gave me that information] used some other words, [but] I'm not going to repeat them until we get more information because of the sensitivities."
Then, along with the AP and Fox News, CNN had another scoop - authorities had the suspect in custody, they were hauling him to a federal courthouse. But the two news outlets seemingly had leapt from the realm of fact to fiction, as dozens of reporters discovered when they tried to follow the story - all they found on arriving at the court building was that it was being evacuated because of a bomb scare.
Similarly, the images released late on Thursday by the FBI might have been the cause of consternation at Murdoch's Post newsroom. They bore no resemblance to the two men who were pictured on page one of Thursday's edition - as men the FBI was interested in.
In a polar opposite to CNN's certainty, The New York Times quoted a senior law enforcement officials: "It's still very squishy ... but most interpretations support the notion that one man is seen dropping a bag."
But more than 1000 investigators on the case seemingly didn't have a name for that individual and, as of late Thursday in Boston, they were still attempting to isolate a facial image of sufficient quality to release as part of a public appeal for information.
An image of the right quality might also be tested against an FBI database that holds 12 million mug shots or be programmed into airport and other transit-system security cameras to help apprehend the suspect if he was on the move.
The week already was redolent with memories of the attacks on New York and Washington in September 2001, when, on Tuesday and Wednesday, the anthrax scare that unfolded in its wake - with five dead and 17 others infected - was resurrected with a ricin poisoning scare in Washington. It was revealed that what were believed to be letters laced with ricin, a lethal toxin, had been intercepted before reaching the White House and the office of a Mississippi Republican senator.
Americans might have been forgiven for believing the ricin letters and the Boston bombing were a low-budget jihadist effort to remind them of all elements of the nightmare of 2001. But the arrest on Wednesday of a Mississippi man accused of sending the letters, to draw attention to his claims of a black market in body parts, put paid to that one.
Like the anthrax plot of 2001, it seemed that the ricin plot of 2013 was home-grown. And while all spoke with caveats and qualifications, speculation by a good number of security experts was leaning towards the Boston attack being home-grown, too.
Apart from the media malaise, the other institutional breakdown this week was in the US Senate. At a time when Americans might have expected their leaders to make a stand against violence, a majority in the Senate, mostly Republicans but with a handful of Democrats and virtually all of whom are in the pocket of the National Rifle Association, voted down a package of gun-law reforms negotiated as a response to the December massacre of 26 schoolchildren and their teachers in Connecticut - and the gun-related death since then of more than 3500 Americans.
In a TV address, an enraged Mr Obama said: "It's well known that 90 per cent of the American people support universal background checks that make it harder for a dangerous person to buy a gun ... and a few minutes ago, 90 per cent of Democrats in the Senate just voted for that idea. But it's not going to happen because 90 per cent of Republicans in the Senate just voted against that idea."
Former CIA terror analyst Larry Johnson told MSNBC TV that the sequencing of the Boston bombs, their assembly and triggering collectively were a hallmark of al-Qaeda, placing the attack "closer to organised terrorism groups than to a crackpot [individual]".
After a Tuesday briefing by intelligence agencies, Republican senator Saxby Chambliss told reporters: "There are a lot of things that are surrounding this that build an indication that it may have been a domestic terrorist."
None of which gives comfort to The Boston Globe columnist Kevin Cullen, who wrote: "We are left with this unnerving proposition: If it was home-grown, it was probably an aberration, the work of a lunatic. If it was foreign-inspired or sponsored, we will never feel safe again in our own town."