CHICAGO (AP) — Federal safety officials said Friday they have launched an investigation into a Chicago commuter train crash that injured nearly 40 people but they’re likely more than a year away from releasing any conclusions about what caused the incident.

National Transportation Safety Board Chairperson Jennifer Homendy said during a short news conference Friday that a team of seven investigators from her agency has started documenting the scene of the crash. They expect to spend about five days on the scene.

The investigators have not reviewed event recorders or video of the incident yet, she said. The agency expects to issue a preliminary report in a matter of weeks, she said, but didn’t specify when. The agency will likely need a year to 18 months to produce a final report with an analysis of what happened, conclusions and recommendations, Homendy said.

The Chicago Transit Authority train was heading south from Skokie on Thursday morning when it rear-ended snow-removal equipment that was moving ahead of it on the same tracks. Thirty-eight people were hurt; 23 were taken to area hospitals. No one suffered life-threatening injuries, according to paramedics.

It remains unclear whether the train operator saw the equipment and tried to slow down or stop the train before the collision. It’s also unclear whether the operator had been warned the equipment would be on the tracks.

Homendy had no details Friday on the train’s speed or what may have precipitated the crash.

Jim Southworth, the NTSB investigator leading the probe into the crash, said the train was equipped with an automatic train control system that’s designed to help prevent collisions by stopping the train in certain circumstances. Homendy said investigators will look into what role, if any, the system played in the crash.

The NTSB has said that system isn’t as comprehensive as the newer automatic braking system known as Positive Train Control. Congress required most railroads to install Positive Train Control in the wake of a 2008 collision between a commuter and freight train in California that killed 25 and injured more than 100.