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 2007-12 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, appealing to people who like its rugged looks and want to go off-road. 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. The company, which is majority owned by Italian automaker Fiat, is making a remarkable comeback from its 2009 bankruptcy and restructuring. It posted a profit last year for the first time since 1997.

The safety agency said on its website Wednesday that it has received 23 complaints about fires in Wranglers from the six model years. Four people were hurt, including three who received minor burns and one whose injuries were not explained in the complaints. Two houses were damaged.

The agency is focusing on overheated transmission fluid and electrical wiring as possible causes. It has asked Chrysler for information about allegations of smoke or fire in Wrangler engine compartments. The company has until May 22 to respond. It's common for the agency to expand investigations to include similar vehicles. The investigation could lead to a recall, but there isn't one yet.

Chrysler spokesman Eric Mayne said in an email that the company was aware of a small number of fires in Wranglers and is cooperating with the investigation. Vehicle fires, he says, are complex and can occur for many reasons unrelated to the vehicle's design and manufacture.