Story highlights Three days after the Boston marathon bombings, the FBI showed images of the suspects

Hours later, a massive shootout unfolded in Watertown between police and the suspects

Almost a month after the terror attack, police sources have shed new light on the Watertown shooting

In the days after last month's Boston marathon bombings, the city was on edge. Residents were holed up in their homes, under strict orders not to leave. Investigators sifted through countless hours of surveillance images trying to determine who might have carried out this heinous attack.

Then came a breakthrough.

Three days after the April 15 attack, the FBI identified the bombing suspects captured in surveillance images near the finish line, later identified as Tamerlan and Dzhokhar Tsarnaev.

And a manhunt was on.

Hours after that FBI news conference, Boston and the rest of the country followed as the suspects allegedly shot and killed a Massachusetts Institute of Technology police officer in a failed attempt to seize his gun, carjacked a black Mercedes SUV whose driver cleverly escaped at a gas station , then fled to the Boston suburb of Watertown, where they were confronted by police.

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Now, almost a month later, two police sources have shed new light on exactly what happened when police confronted the two suspects in Watertown during the early hours of April 19, triggering a gun battle unlike anything recently seen on the streets of an American city.

Among the new details from the two police sources:

-- Police fired nearly 300 rounds of ammunition within five to 10 minutes as they confronted the suspects -- 100 more than initially reported. And that included one round that nearly killed Massachusetts Transit Police Officer Richard Donohue. (Others bullets struck the Tsarnaev brothers, seriously injuring Dzhokhar and contributing to the death of Tamerlan.)

-- Tamerlan was the only brother armed with a handgun. The only other weapons they had were the homemade explosives that police say the brothers tossed out of the hijacked vehicle, including a few that detonated.

-- Police accidentally fired on an unoccupied black SUV during the mayhem. "In the chaos, an officer or trooper (or some combination of personnel) mistook it for one of the two suspect vehicles," David Procopio of the Massachusetts State Police told CNN.

In the beginning, all police knew just before 11 p.m. Thursday, April 19, was that a man was down in Cambridge. They would later learn that MIT campus police officer Sean Collier had been shot and killed in his patrol car.

A short time later, at least half a dozen police agencies responded to a call from help from the Watertown Police Department, after an officer spotted two cars that the Tsarnaevs were driving.

The officers converged on the narrow residential intersection of Dexter and Laurel streets in Watertown. The Tsarnaev brothers were in the middle of the street, after lobbing the homemade explosives.

Law enforcement sources told CNN that the only other weapon the brothers had was a single handgun used by Tamerlan, but investigators have not confirmed that.

"We have not commented publicly on that and I'm not in a position to do so at this time," Procopio said. "Obviously, that is part of the investigation."

The extraordinary gun battle played out in front of Andrew Kitzenberg, who captured photos of the scene on his iPhone from his upstairs apartment on Dexter Street.

In one image taken by Kitzenberg, the two suspects are crouched behind the black SUV as they engage in gunfire. Kitzenberg said the two men had been transferring bags from a green sedan that one of the brothers drove to the scene.

At one point, Tamerlan approached police on foot, with explosives on his body and an explosive trigger, firing his handgun at the officers who returned fire, mortally wounding him. A police officer tackled him, then handcuffed him on the ground. Around the same time, Dzhokhar grabbed the wheel of the hijacked SUV and drove toward the police.

"As soon as the SUV turned around in the street," Kitzenberg told CNN's Drew Griffin, "it was accelerated gunfire."

At some point, Dzhokhar was shot and wounded. As he drove the SUV toward the officers who had restrained his brother, the police scrambled out of the way and Dzhokhar proceeds to drive over his brother.

Tamerlan later dies from the gunshot wound and the blunt force trauma of being run over, according to his death certificate

Several hours later, a seriously injured Dzhokhar was arrested a few blocks away in Watertown, where he was hiding underneath a covered boat in someone's backyard.

Massachusetts State Police and the Middlesex District Attorney's office are investigating the entire sequence of events that night, beginning with Collier's death in Cambridge.

"That shoot review, which is ongoing, will include examination of any potential friendly fire incidents," Procopio said. While Donohue was the only officer seriously injured in the Watertown shooting, another officer was also grazed by a bullet.

In high-pressure situations, police often experience "contagion shooting," a former Branford, Connecticut, police chief said. And that's most likely what happened in Watertown.

"If you look back at some of the other cases in the past, then it immediately causes a contagion and other people start shooting," said John DeCarlo, now an associate professor of criminal justice at the University of New Haven.

The gunfight in Watertown was so intense that bullets came flying through a home about a half a block from the shooting scene.

"They landed right near our staircase, near the pedestal and the other near our closet," said Harry Ohannesian, who showed CNN several bullet holes in his house. "They went through one, exited, went through another closet and landed under the staircase."

The chaos in Watertown that led to the friendly fire shooting of Donohue can be compared to a war zone, DeCarlo explained.

"Things (that) occur in the very dynamic moments of a situation like this ... are not necessarily -- no matter how hard police work -- what they are trained to do," said the former police chief, noting that none of his remarks are intended as criticism of police.

Procopio concurred with that comparison:

"Considering the chaos and the battlefield conditions -- where a pair of homicidal terrorists were firing shots and throwing bombs at police -- the fact that friendly fire incidents might have occurred detracts nothing -- not one bit -- from the valor exhibited by the police officers and state troopers present that night," the Massachusetts State Police spokesman said.