Moments after investigators went before television cameras to broadcast photos of the two men in ball caps wanted for the Boston Marathon bombing, queries from viewers started cascading in — 300,000 hits a minute that overwhelmed the FBI's website.
It marked a key turning point in a search that, for all the intensity of its first 72 hours, had failed to locate the suspects. While it's unclear how much the tips that resulted helped investigators zero in, experts say it instantly turned up already intense pressure on the two men to flee or almost certainly be recognized — increasing the chances they'd make mistakes that would lead to them being exposed.
The decision to ask the public for help also was something of a gamble, one that investigators had to weigh carefully.
"It was a good decision to put this out to the public ... and this would have been a calculated risk. But the intent would have been to get these guys to change their pattern" of behavior, said Martin Reardon, who spent 21 years as an FBI agent and is now a vice president of security consultant The Soufan Group.
Releasing the photos greatly increased the odds the two men would be recognized and turned in, even as it significantly upped the chances they would try to vanish or commit more mayhem — exactly the scenario that has played out.
"Clearly these guys were reacting and responding exactly as (law enforcement) predicted," said Robert Taylor, a criminologist at the University of Texas at Dallas who studies terrorism. "If you saw your face on TV and everywhere else as associated with the bombing ... you would act irrationally, and that's exactly what they did."
After three days without being able to identify a suspect by name, investigators clearly made the decision to release the photos Thursday on the belief that, without doing so, the suspects might remain at large for weeks or months, with the chance to flee or to act again, said David Weinstein, a former federal prosecutor in Miami.
So with photos in hand, investigators made a choice deemed both necessary and prudent.
"And then the worst possible thing happens," Weinstein said. "They do actually begin their flight and then start to wreak vengeance on the whole city of Boston."
Weinstein, Reardon and other experts had differing opinions on whether investigators' decision to release the photos was worth the cost exacted by the two men: the killing of a Massachusetts Institute of Technology police officer, a carjacking, the shooting of another transit police officer and a block-by-block manhunt that led officials to shut down Boston and many of its surrounding suburbs.
But all agreed the photo release was pivotal in breaking open the case, because it instantly deprived suspected bombers Tamerlan Tsarnaev, 26, and his 19-year-old brother, Dzhokhar, of time, anonymity and options.
By late Friday, many of the details in the chain of events that led to the older brother's death and the younger one's capture were still unclear, but the pursuit had consumed the region with apprehension — apprehension that turned to jubilation when Dzhokhar Tsarnaev was captured Friday night.
It began just after 5 p.m. Thursday, when investigators released the photographs and video of two unidentified suspects and asked for the public's help. Just over five hours later, shots were heard on the campus of the Massachusetts Institute of Technology, across the Charles River from Boston in Cambridge. Ten minutes later, an MIT campus police officer who was responding was found shot multiple times in his vehicle and was later pronounced dead.
Soon after, two armed men carjacked a Mercedes SUV in Cambridge, holding the driver for about half an hour before releasing him unharmed. Police pursued and the men inside the vehicle threw explosive devices from the windows, while exchanging gunfire. A Massachusetts Bay Transportation Authority officer was wounded in a firefight with the suspects and the older brother, Tamerlan Tsarnaev, was critically injured and pronounced dead.
The hunt then continued for the younger brother, who fled on foot.
In the pre-dawn hours Friday, dozens of police officers and FBI agents converged on Watertown, Mass., after gunshots and explosions were heard, ordering people to stay inside. But the search proved fruitless, leading authorities to shut down Boston's mass transit system and urge residents of several cities and towns to stay indoors.
State Police spokesman Dave Procopio said police realized they were dealing with the bombing suspects based on what the two men told the carjacking victim during their getaway attempt.
"We believe this man to be a terrorist," Boston Police Commissioner Ed Davis said, of the brother who remained a fugitive late Friday. "We believe this to be a man who's come here to kill people."
Authorities eventually caught 19-year-old Tsarnaev with a combination of old-fashioned vigilance from the public and impressive technology from the police.
Late Friday, a man in Watertown noticed something unusual about a boat trailered in a back yard: There was blood near it. He approached the boat, peaked in, and saw a body covered in blood, authorities said.
He reported it to police, who then used helicopters equipped with thermal imagers to check for a heat signature inside the tarp-covered boat. The thermal readings confirmed that there was someone under the tarp, and that the person was still alive.
"He wasn't moving at first," one police official told NBC News. "He is now."
After monitoring the body in the boat for more than an hour, police moved in and seized Tsarnaev, the wounded bombing suspect.
The chaos of the pursuit contrasted sharply with the sweeping, methodical investigation that began almost immediately after the Monday afternoon bombing that killed three and wounded more than 180, marked by officials' notable reluctance to disclose information. In the hours and days immediately after the bombing, dozens of investigators in white hooded suits carefully combed, cataloged and photographed evidence at the scene, even canvassing the roofs of nearby buildings to search for items blown into the air by the bomb's force.
Investigators gathered hours of videotape footage from security cameras that scanned the area around the bombing and appealed to the public to turn in their own video and photos, for help in determining the sequence of events and identifying a suspect.
They then used software to search for certain types of objects or people matching a height and weight description. The software can also spot patterns that human analysts might not notice, such as a car that turns up in different places, said Gene Grindstaff, a scientist at Intergraph Corp., a company that makes video analysis software used by the FBI and other law enforcement agencies.
"Back in the days of 20 years ago, you were lucky if you had video and it was probably of poor quality and it took a tremendous amount of enhancement. Today you have a completely different issue," Grindstaff said.
"Here's the first thing that the computer was told: Tell me if you can find the same people at both of those (bombing) locations," said Taylor, the criminologist.
Additional parameters would further narrow the search to, for example, look for people carrying backpacks.
"It's kind of like going through a series of strainers and filters," Taylor said.
But with the video winnowed down, the process required examination frame-by-frame, a laborious process done by an FBI unit called the Operational Technologies Division, said Joe DiZinno, former director of the FBI lab in Virginia.
By Thursday, once facial recognition software and agents had narrowed the search to images of two young men, investigators had to make a decision about how to proceed.
Meanwhile, the Tsarnaev brothers were already on edge.
At an auto body shop near their home, the younger brother, Dzhokhar, had often stopped to talk with owner Gilberto Junior about cars and soccer. But on Tuesday, the day after the bombing, the normally relaxed young man showed up biting his nails and trembling, Junior said.
The mechanic told Tsarnaev he hadn't had a chance to work on a Mercedes he'd dropped off for bumper work.
"I don't care. I don't care. I need the car right now," Junior says Dzhokhar Tsarnaev told him.
By Thursday afternoon, the brothers had to know their options were narrowing quickly. And then the FBI released their photos to millions of viewers across the city, and around the world via newspaper, television stations and websites. The time to move was now.
"I think this developed rather quickly last night," State Police Col. Timothy Alben said late Friday. "I would wager that most of the activity that was printed in the media yesterday forced them to make decisions or take actions that ultimately revealed who they were."
NBC News Science Editor Alan Boyle contributed to this report.