WASHINGTON — Federal safety regulators said Saturday they have upgraded an investigation into 600,000 Chrysler minivans after reviewing more than 1,500 complaints that the headlights went out without warning.

The National Highway Traffic Safety Administration said in a notice posted Saturday on its website that it upgraded an investigation to an engineering analysis into 622,817 2005 model Chrysler minivans — including the Town and Country, Voyager, Dodge Caravan and Dodge Grand Caravan, according to The Detroit News.

NHTSA said the complaints report the headlights go out without warning — in most cases while the vehicle is in motion — and that "in cases where repairs have been attempted which consisted mainly of headlight switch and body control module replacements, mixed success is reported."

Owners have said they have been able to use high beams when the headlights went out. NHTSA says no crashes or injuries have been confirmed to be connected to loss of headlights. NHTSA first opened a preliminary analysis into the issue in July 2010.

Chrysler Group LLC said it has one report of an owner saying he lost lights and struck a deer, and a second in which a driver said he lost control and drove into a ditch. Both occurred several years ago and NHTSA hasn't confirmed either one.

Chrysler's senior manager of product investigations and campaigns, David D. Dillon, told NHTSA the company's analysis of complaints "indicates a declining trend."

"There is no evidence to suggest this trend will reverse," he told NHTSA in a letter. "The lack of harmful events being reported supports the conclusion that there is no demonstrative risk to motor vehicle safety."

Chrysler also told NHTSA it has received 14,208 warranty claims that may be related to the condition. In total, NHTSA has reviewed 1,541 complaints of problems with headlights.