Federal safety regulators have stepped up their investigation into Jeep Liberty air bags after 50 people reported they were hurt when the air bags inflated without a crash happening.

The National Highway Traffic Safety Administration started investigating Liberty SUVs from the 2002 and 2003 model years in September. It was upgraded to a full engineering analysis last week.

Documents on the agency website say Chrysler and regulators have
gotten 87 complaints of air bags going off by surprise. Nearly 387,000 vehicles are under investigation.

Drivers reported burns, cuts and bruises.

Safety regulators say the air bag computer may fail due to an electrical voltage spike.

The company says no incidents have happened in vehicles made after March 19, 2003. But regulators say Chrysler can't explain that.