BOSTON -- National Transportation Security Board and Massachusetts Bay Transportation Authority investigators continued to work Monday to examine the cause of a Green Line trolley crash that injured close to 50 people Friday night.

The MBTA announced a tough new cell phone policy Saturday after discovering the crash, which happened after a trolley operated by Aiden Quinn, 24, who was texting his girlfriend, collided with another trolley between the Government Center and Park Street stations.

At least nine other workers have been suspended for texting while driving since October.

"If a zero tolerance policy on use of cell phones isn't sticking with our employees, then maybe we need to go to the next level," Grabauskas said.

Grabauskas said that next level is banning the possession of cell phones by train operators while on duty.

Close to 50 passengers suffered minor injuries in the crash and were transported to area hospitals. The effect is clear and so is the cause, investigators said.

Grabauskas said Quinn texted his girlfriend after he was unable to reach her by phone.

"He was texting his girlfriend,” he said. “That is what I have been told by the detectives and that there was a phone call that he attempted to make just prior to that. The phone call did not go through; he attempted then to reach her by text.”

Trolley operators were already banned from texting, but Friday’s crash has triggered an even tougher new policy that will soon take effect: If train drivers are caught with a cell phone while working, they will be fired.

The NTSB has not yet determined the probable cause of the crash, NTSB spokeswoman Debbie Hersman said.

“We’re going to leave no stone unturned,” she said. “We have some very good information obviously from the operator.”

The Government Center station remained sealed off Saturday as the National Transportation Security Board conducted a formal investigation.