Three of four Swampscott young men in the car that crashed into the stone bridge abutment on Stetson Avenue early Saturday morning are now home.
The fourth patient remains hospitalized.
Meanwhile, State Police are investigating why the operator of the 2013 Chrysler 300 crashed.
Troop A of the State Police is conducting the investigation with help from the agency's Collision Analysis and Reconstruction Section, said David Procopio, spokesperson for State Police.
State police is handling the investigation at the request of Swampscott police, he said.
The driver of the vehicle has a familial relationship to the Swampscott police chief, the spokesman said.
"It is something MSP does for local departments periodically, upon their request, to avoid even the appearance of a conflict of interest," the spokesman said. "It is the proper course of action and Chief Madigan should be commended for initiating it."
The spokesman said he will release the names of the accident victims when he is able to get the information from the State Police sergeant who is leading the investigation.
The crash reconstruction is ongoing.
At this point the investigation has determined that the vehicle was traveling southwest on Stetson approaching Norfolk Street about 12:30 a.m. Saturday. "When the vehicle reached a curve to the left, the operator failed to negotiate the curb, traveled onto the sidewalk and the grass strip adjacent to the sidewalk, and impacted the wall of an abandoned granite railroad bridge," the spokesman stated.
The engine compartment caught fire and the front seat passenger was removed by area residents and was transported by medical helicopter to Mass. General with life-threatening injuries.
A rear-seat passenger suffered serious injuries and was transported by ambulance to Brigham & Womens, the spokesman said.
That patient was originally taken to Salem Hospital before being transferred to Brighham and Women's, police said earlier.
The driver and a second rear-seat passenger were transported by ambulance with non-life threatening injuries .
They, too, were transported to Salem Hospital, according to an earlier report.
The four people in the car were in their early 20s and from Swampscott, Swampscott police Det. Tim Cassidy said earlier.
The detective said on Monday that he had learned that the patient who was transported to Brigham and Women's was released from the hospital and at home.
The entire crash analysis usually takes a few weeks to complete, the spokesman said.
Swampscott fire and police units responded to the crash, Fire Chief Kevin Breen said earlier.
All of the individuals were conscious upon the arrival of first responders, the fire chief said earlier.
All the individuals were out of the vehicle, which caught fire, he said.
Fire crews extinguished the fire.