Two people were critically injured after a gunman using an assault-style rifle began shooting into traffic in Cambridge, Mass., on Monday afternoon, authorities said.

The suspect, identified as Tyler Brown, a 46-year-old Boston resident, was apprehended by officials following a shootout, according to Middlesex District Attorney Marian Ryan.

As drivers and passengers fled the gunfire, a state trooper and civilian — whom authorities identified only as a former Marine licensed to carry a firearm — ran toward the shooting and exchanged gunfire with the suspect, striking him in the extremities, officials said.

Two men in separate vehicles were shot and are currently being treated for life-threatening injuries at Boston hospitals. Ms. Ryan said the trooper’s cruiser was also struck by gunfire.

Authorities did not identify a motive in the shooting.

“This incident lasted minutes thanks to the actions of the trooper and that civilian,” Ms. Ryan said at a news briefing on Monday evening.

The gunfire erupted near the River Street Bridge along Memorial Drive, a heavily traveled road that runs beside the Charles River. The area is close to Harvard University in Cambridge and the Massachusetts Institute of Technology in Boston.

Ms. Ryan described the weapon used by the gunman as an assault-style rifle.

“We know that that weapon had the capacity to have struck people on the other side of that river,” Ryan said.

She later added: “I would venture to guess that there is no one in this room who hasn’t had occasion to travel along that stretch of Memorial Drive.”

Mr. Brown was being treated for gunshot wounds at a hospital. He will be charged with two counts of armed assault with the intent to murder, among other charges, Ms. Ryan said.

After receiving reports of gunfire around 1 p.m., Cambridge Police and Massachusetts State Police arrived to find an active shooter firing “in an erratic fashion” at vehicles along the road, officials said. In an audio recording played at the briefing, several rounds of gunfire could be heard over the course of a minute.

Video broadcast by CBS Boston showed a man carrying a gun, pointing it indiscriminately and waving the weapon as he walked along the street.

The gunman fired as many as 60 rounds in a matter of minutes, and a least a dozen vehicles were damaged, officials said. Drivers and passengers abandoned their vehicles and fled the scene, some hiding beneath their cars. There was no indication the gunman knew any of the victims, officials said.