The National Transportation Safety Board Sunday continued investigating what caused a Green Line trolley to collide with another one Friday evening along Commonwealth Avenue and is expected to release a preliminary report on the crash in a few weeks, according to an NTSB spokeswoman.
The collision left 25 people with non-life-threatening injuries, and a Massachusetts Bay Transit Authority subway operator was placed on paid administrative leave due to developments in the investigation, officials said Saturday.
Four NTSB investigators were on scene Sunday, said spokeswoman Jennifer Gabris, who added that their “specialties include operations, crashworthiness, and human performance.”
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Federal investigators were gathering information, including how fast the train was traveling when it struck the other trolley, according to Gabris. “Parties to the investigation” have been identified, she said.
“At this early stage of an investigation, NTSB does not state a cause but will provide factual information when available,” she said in an e-mail to the Globe. “Investigations involving injuries and other major investigations currently take between 12 and 24 months to complete.”
She directed other questions about the investigation to local authorities.
Suffolk District Attorney’s office spokesman Matthew Brelis said Sunday only that the investigation was active and ongoing.
The operator placed on leave has been with the MBTA for seven years, MBTA spokesman Joe Pesaturo said Saturday. That employee had been at the controls of the first car of the two-car trolley which crashed into the other Green Line train Friday.
Pesaturo referred questions, including those about the train operator, to the NTSB Sunday. Gabris did not release any information about the operator, however, including an identification of the person.
“NTSB does not release the names of those involved in the accidents we investigate,” Gabris said.
Friday’s crash occurred when a westbound MBTA B branch train struck another train from behind shortly after 6 p.m. near the corner of Commonwealth Avenue and Pleasant Street.
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After the crash, a woman who was apparently an MBTA employee called for help over the radio, and told the agency’s dispatcher that the train she was on had been hit “head-on” from behind.
“I have passengers who are bleeding and injured,” the woman said. “Get here as soon as possible, please.”
A dispatcher replied: “We have help on the way, ok? We have help on the way.”
Among those injured were four train operators, officials have said.
Many of the passengers injured were transported to Massachusetts General Hospital and St. Elizabeth’s Medical Center following the collision, which damaged both trains and the tracks.
An MGH spokeswoman Saturday said five people sent to that hospital from the crash were treated and released. Representatives for St. Elizabeth’s and Steward Health Care did not respond to messages over the weekend.
Train service in the area resumed at 6:20 a.m. Saturday.
Friday’s crash follows two other collisions on the Green Line that drew in the NTSB, including a 2008 incident in Newton when a MBTA subway operator was killed after she passed a stop signal and struck a stopped train in front of her. In that crash, the NTSB concluded that Ter’rese Edmonds, 24, suffered from fatigue and could have briefly fallen asleep.
The following year, Green Line train operator Aiden Quinn crashed his train into another trolley near Government Center. The NTSB determined Quinn had been texting on his cellphone when the crash occurred. Following that crash, the MBTA banned operators from carrying phones while at work.
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Gabris said examination of a subway operator’s phone is typically part of any NTSB investigation.
The MBTA also requires that a subway operator undergo drug and alcohol tests after a crash, according to the agency.
After the 2008 crash in Newton, the NTSB faulted the MBTA for not implementing a safety system to help prevent train collisions. Officials criticized the transit agency over the same issue following the crash near Government Center.
The technology, intended to stop a train if it approaches an obstacle too quickly, has been installed, however, on the MBTA’s Blue, Orange, and Red lines, and on the Commuter Rail.
On Saturday, Pesaturo said installation of the $170 million Green Line Train Protection System is expected to begin early next year.
Laura Crimaldi of the Globe staff contributed to this report.
John Hilliard can be reached at john.hilliard@globe.com.