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."
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."
Associated Press writers Curt Anderson in Miami, Jeff Donn in Boston and Jay Lindsay in Watertown, Mass., contributed to this report.
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