DETROIT — U.S. safety regulators have expanded an investigation into 23 complaints of fires in Jeep Wrangler SUVs.

The National Highway Traffic Safety Administration hasn’t figured out what caused the fires, but it’s asking carmaker Chrysler for information on Wranglers from the 2007 through 2012 model years. When it began March 28, the probe centered on vehicles from the 2010 model year.

The classic Wrangler is among Chrysler’s more popular models. Chrysler sold more than 532,000 Wranglers from 2007 through March. It’s unknown how many are affected by the investigation.

The probe comes at a critical time for Chrysler, which has had quality problems in the past but showed improvement in an annual study by Consumer Reports magazine.