WASHINGTON -- With thousands of tips pouring in from the public, investigators were homing in on the Boston Marathon bombing suspects on Thursday morning -- or so they thought.
By that afternoon, however, the promising leads had collapsed, and officials confronted a risky decision: proceed without the help of the public to avoid tipping off the suspects or publicize images of them and risk driving them deeper into hiding or worse.
FBI officials, who had been debating all week whether to go to the public, were ultimately convinced that they had to release the photographs because the investigation was stalling and bureau analysts had finally developed clear images of the suspects from hours of video footage.
"We didn't want to drive them underground," a senior law enforcement official said. "We ran down a lot of leads and tips, and we still couldn't identify them. We were working the videos, and the footage was getting better and better as the week went on, and by Thursday we got a good frontal facial shot. That tipped it."
The official added: "With that type of quality photo, there was no doubt about who they were. We had these murderers on the loose, and we couldn't hold back and we needed help finding them."
The decision -- which involved Attorney General Eric H. Holder Jr. and Robert S. Mueller III, the director of the FBI -- was one of the most crucial turning points in a remarkable crowd-sourcing manhunt for the plotters of a bombing that killed three people and wounded more than 170.
While the decision to publicize the suspects' identities ultimately resulted in the end of the manhunt, it set in motion a violent string of events that lasted for 26 hours. Over that time, a police officer was killed, one of the suspects died, several officers suffered life-threatening injuries and one of the country's major cities was shut down.
The authorities knew that broadly distributing the images -- some captured by ubiquitous surveillance cameras and cellphone snapshots and winnowed down using sophisticated facial-recognition software -- would accelerate the digital dragnet, but they did not realize the level of chaos it would create.
Intelligence and law enforcement officials said the authorities in Boston weighed the risks of some mayhem against their growing fear that time was slipping away, and that heavily armed and increasingly dangerous men, and possibly accomplices, could wage new attacks in the Boston area or beyond.
"Their posting of that video and photographs speaks to the fact they likely felt they had run into a dead end," said Mitchell D. Silber, a former senior intelligence official in the New York Police Department who is now with the investigative firm K2 Intelligence.
Federal authorities involved in the case had briefed administration and congressional officials on their hopes to arrest the suspects early Thursday without revealing their hand.
"We thought we had good leads," one senior law enforcement official said. "We were working on some stuff, and we got to a point where it leveled off, and then there was nothing imminent, so we moved with what we thought would result in identifying them."
The authorities first developed information about the suspects' whereabouts late Thursday night when one of them was seen in video footage from a convenience store in Cambridge that had just been robbed.
Shortly after the suspects left the store, the authorities received a report that a police officer at Massachusetts Institute of Technology had been ambushed and killed.
And then, for two hours, there was no sign of the suspects.
It was only after the brothers decided not to kill the owner of a sport utility vehicle that had been carjacked and instead threw him out of his car around 1 a.m. that the suspects re-emerged on the authorities' radar.
The driver had called 911 shortly after he was dumped from his car, telling the authorities that the two people who had held him up at gunpoint had said they were the marathon bombers. The authorities put out an all-points bulletin for the sport utility vehicle, and officers in Watertown -- 10 miles west of Boston -- spotted it just minutes later.
A chaotic chase ensued, with the suspects throwing pipe bombs out of the speeding vehicle. Of the five that were thrown, three exploded, injuring officers.
The suspects were ultimately cornered by the police and engaged in a shootout in the middle of Watertown. Several more officers were hurt, and Tamerlan Tsarnaev, who was strapped with explosives, was killed as he approached the officers.
After Tamerlan Tsarnaev was shot, the younger brother -- who was at that point low on ammunition -- shifted to the driver's seat of the car from the passenger's seat, slammed it in reverse, ran over his brother and sped away. From that point until he was captured roughly 20 hours later, he was again off the radar of the authorities.
Soon after the shootout, officers in Watertown found the sport utility vehicle nearby. It was only at that point -- with one dead suspect and an abandoned vehicle -- that the authorities learned their identities.
The FBI and other investigators began an intensive search of databases, and agents fanned out to interview family members and others who might have known them.
By early Friday morning, Watertown had been transformed into an armed camp, with hundreds of police officers and agents searching house by house.
But by 6 p.m. Friday, there was still no sign of the young fugitive, and Gov. Deval Patrick lifted the restrictions, allowing people to leave their homes.
That order allowed a Watertown resident to go outside for the first time all day, where he spotted blood on a boat in his back yard. He pulled back the tarp on top, peered in and saw a young man covered in blood.
Within minutes, police cars were screaming toward the home. A police helicopter hovering overhead threw its spotlight on the boat. Shots were fired. By 8:45 p.m., Dzhokhar Tsarnaev, who had been wounded at some point in the chaos, was in custody.