As part of an ongoing series, WGBH News explores the unanswered questions that surround the massive manhunt in Watertown, Mass. on Friday, April 18 and the ultimate capture of Boston Marathon bombing suspect Dzhokhar Tsarnaev. This is the second story of a four part series.

At 10:31 p.m. on April 18, MIT Police Officer Sean Collier was fighting for his life after being shot multiple times while sitting in his cruiser. Fellow officers rushed to his side trying to stem the bleeding.

Lt. Jeremy Walsh, of the Cambridge Fire Department, was among the first to respond at Vasser and Main Streets.

"I was in the firehouse when the call came in, roughly 10:20, 10:30," Walsh said. "There were some police officers in the scene who had started care and we relieved them. It was our job to get him on a gurney; on a board into the ambulance."

Within two hours of treating officer Collier in Cambridge, Jeremy Walsh would be called to Watertown in response to the shooting of another officer — MBTA Police Officer Richard Donohue.

"That’s what made that the longest night of my career," Walsh said.

The circumstances seem uncanny. Donohue was a close friend of officer Sean Collier.

"The talk in Watertown — just a few hours later, was that they knew each other," Walsh said. "That they were academy mates And that they were shot on the same night. It was just another 'Oh my God, I can’t believe this is happening' moment. I mean, yeah, it was a strange coincidence, and maybe someone is looking over Officer Donohue, cause the injuries that he sustained, people rarely survive those injuries."

Donahue was hit around 1 a.m., and he was pulled by fellow officers into Jeffrey Ryan’s driveway:

"He was shot here, and they pulled him into the driveway to get him out of the line of fire, which was still going on over that way," Ryan said. "So I came out, and I saw these three policemen, doing emergency response on this fellow, and they were frantic. First of all, he'd been shot in the femoral artery, and blood was like a geyser, it was just shooting everywhere. They said, 'Towels, towels, towels! Get us towels for tourniquets.' And so I shouted to my wife, who sent a bucket of towels down the stairway, and I brought them out and they used that to stanch the bleeding."

Who shot Donohue? Six months later we still do not know. However, when you look into the state charges against Dzhokhar Tsarneav, it includes the murder of Collier, and armed assault with intent to murder. But the charges against Tsarneav do not include the wounding of Donahue. Does this omission suggest he was shot by friendly fire? When we asked Watertown chief Ed Deveau, he said it’s still much too early to know.

"I don't know," Deveau said. "It's still early on as we go through everything that happened at that point. We kind of have some idea of what was happening before, and after, but my officers were in that gunfight, so I'm not sure, until we piece everything together, what happened and when it happened."

Moments before Donahue was shot, police were involved in a furious gunfight with Tamerlan and Dzhokhar Tsarnaev on Laurel Street.

At one point during the shootout at just before 1 a.m. Dzhohar Tsarnaev made a run for the SUV while his brother advanced on the police on foot. Former Watertown Resident Andrew Kitzenberg watched from a third floor window.

"As one of the brothers was running down the street, as he got closer to the officers, that's when I saw the SUV turn around in the middle of the street and accelerate toward those police vehicles," Kitzenberg said.

Laurel Street Resident Mike Doucette picks up the story from there.

"They were shooting up the car as he tried to run over the two cops, when they jumped, and he r"n over his brother, Doucette said. "And there was a cruiser right here — he rammed that cruiser with the front left bumper. That's when the brother came rolling out from underneath the car. And he hit that, and that's when they all started shooting at him, about right here, before he went through the intersection. That's when they shot the other cop in the leg."

I asked Doucette who he was referring to when he said, "they shot the other cop."

"The cops shot the cop," he said.

The situation at that moment was described as "chaotic" by several witnesses interviewed by WGBH News. A law enforcement source says the fast-moving situation was made worse by the sudden arrival of police officers who showed up in Watertown without being called — so called self-deployment. Another source who specializes in police policy and training — speaking under the condition of anonymity — says it is a miracle no one else was shot that night.

"We had so many self-deployed officers, we don’t know where the bullets from all of those guns came from," the source said. "In such a confused set of circumstances, with so many local, state and federal police folks there, there was very little coordination, and each of those agencies have a different set of policies and different set of training, so altogether it’s amazing no innocent person was killed. It truly was."

Deveau says the chaos of that evening should be put in perspective.

"I don't think any scene is going to be really controlled in the first minutes," he said. "You just have so much going on. When something happens, people want to be there. The media want to be there. So everybody wants to do their job and they truly want to help. I think we just need to be able to make sure we can put that help to work in the right ways."

WGBH News requested officer reports from Watertown and the MBTA based on the state’s public records statute. Both requests were denied, we were told, due to the ongoing investigations. WGBH News is appealing to the Secretary of the Commonwealth for access to that information.

The issue of officers self-deployment in Watertown is being discussed by Homeland Security. Boston Police Commissioner Ed Davis, Cambridge Police Commissioner Robert Haas and Deveau recently flew to Washington to meet with Homeland Security Sec Janet Napolitano. Cambridge Police Commissioner Robert Haas spoke to WGBH News about the content of that discussion.

"And one of the issues we identified during the course of that conversation was the notion of self-deployment; this appetite, this thirst for officers wanting to get engaged over getting involved, but it also can cause an additional complication in terms of having officers respond, not having a structured hierarchy in which you can actually start to relegate or delegate people in terms of how you do things,” Haas said.

Another question raised at the meeting with Homeland Security officials was: How did Dzhokhar Tsarnaev elude law enforcement the morning of Friday, April 19, as hundreds of police surrounded the east Watertown neighborhood?

That’s the question we’ll explore next.