JUNEAU, Alaska – The plane that crashed into an Alaskan mountainside Monday and killed former Sen. Ted Stevens and four others was outfitted with an alert system that warned pilots of dangerous terrain.

But National Transportation Safety Board Chairwoman Deborah Hersman said it’s not known if the system was working just before the crash.

The plane was also equipped with an emergency locator transmitter, Hersman said Friday in Anchorage. When properly registered, the transmitter issues a distress signal to a control center via satellites and provides registration information, such as the owner’s name.

Hersman said it was unclear why the device didn’t activate.