BOSTON (AP) — One of the trains involved in a crash on the Boston subway system that sent four workers to the hospital earlier this month did not stop at a red signal and was traveling at speeds higher than the posted limit, an investigator said Monday.

The Green Line train that caused the contact in the June 1 crash was traveling to the Government Center station at approximately 9 mph (14.5 kph), Steven Culp, the Massachusetts Bay Transportation Authority’s chief investigation and safety assurance officer, told the MBTA board’s Safety, Health & Environment Subcommittee.

The other train was traveling at 5.7 mph (9.2 kph), he said. The speed limit in the area is 7 mph, a spokesperson said.

The impact derailed both trains and disrupted service.

Four operators were taken to Massachusetts General Hospital. None of the 20 to 25 passengers on board the train that caused the contact required medical treatment. Per protocol, all four employees were placed on paid administrative leave during the investigation.

The investigation remains ongoing, but the MBTA has issued an “attention to duty” reminder to operators, telling them to remain vigilant about their surroundings and signals, Culp said.

The collision occurred amid a federal review of the MBTA’s safety following several recent accidents that led to injuries or death.

MBTA Chief Safety Officer Ron Ester told the subcommittee that Federal Transit Administration investigators are expected to wrap up their work this week, but have already issued four recommendations. They are making sure control center staffing levels are adequate, ensuring safety of train operations at yards, up-to-date track maintenance and current operator certifications.

“We’ve had multiple conversations with FTA on these matters and are already at work on mitigations that we will share with the FTA shortly,” Ester said.

The agency has had several safety problems in recent months. A 39-year-old man died in April when he was dragged after his arm got stuck in a malfunctioning subway car door. Nine people were injured in September when an escalator at a station malfunctioned, and more than two dozen people went to the hospital last July when a Green Line train rear-ended another trolley.

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