What evidence will police look at in Memorial Drive shooting? Former Boston Police Commissioner explains.

Massachusetts State Police are collecting evidence at the scene of Monday's shooting on Memorial Drive in Cambridge. According to former Boston Police Commissioner Ed Davis, it's a long and precise process.

The shooting happened around 1:30 p.m. on Monday. Witnesses said that a man opened fire at cars driving along the busy street, forcing them to "run for [their] lives." No Massachusetts State Police troopers were shot, according to I-Team sources.

Davis, who serves as a security analyst for WBZ-TV, explained that police will begin by recovering the weapon and tracing its serial number to determine whether it was stolen and to review any sales history.

"The most important evidence at the scene will obviously be the firearm," Davis explained. "Finding out where the suspect bought the gun to see if there are additional people that are involved that may be charged."

They will also collect any bullet casings around the area to run those through a national database. That can determine if the bullets - and the weapon they were fired from - are tied to weapons used in any previous shootings. Davis explained it will allow police to "get a good idea of who they are dealing with."

"Whether it was a crime gun that was used in other incidents and exactly what the motivation of this guy is," Davis said to WBZ-TV. 

Davis said that investigators will then begin combing through video footage from traffic cameras as well as from security cameras at private businesses and residences. 

"Inevitably, someone will have pulled out their phone and videotaped what was happening. So that is crucial evidence, especially for people who were very close to the action, that can show exactly where the person was coming from, exactly what he was pointing at, and also what he may be saying during the course of the evidence."

Investigators will be taking witnesses' statements on scene before transferring some of those people back to Massachusetts State Police headquarters for longer conversations, according to Davis. 

After that, Davis said that background work will need to be done, including reviewing social media and phone records of the suspect. 

"There's a lot of work that needs to be done and there are really great people that conduct that work," Davis said.