Los Angeles ? Federal officials investigating Friday’s fatal Metrolink train crash in suburban Los Angeles focused Sunday on whether a signal that should have alerted the engineer to stop the train was working properly and if it went unheeded.

National Transportation Safety Board member Kitty Higgins said a computer reading indicated that the last signal before the collision site was displaying a red light. But she said investigators want to make sure that it wasn’t a false reading.

Higgins criticized Metrolink for saying Saturday that an engineer had been at fault for failing to heed the red signal, causing the crash with a Union Pacific freight train that so far has claimed 25 lives and left 135 injured, 40 critically.

The train passed four signals that, if working correctly, would have flashed yellow or red to warn the engineer to slow and stop.