WASHINGTON — The federal government on Wednesday upgraded its investigation into Ford Windstar minivans amid new concerns about corrosion in the vehicles.

The National Highway Traffic Safety Administration said in a posting on its website that it had begun an engineering analysis, an advanced stage in its investigation, of about 550,000 Windstars from the 1999-2003 model years.

Ford has recalled more than 600,000 of the minivans in the U.S. and Canada since August to address rear axles that can corrode and break. The U.S. recall is limited to 22 cold weather states where road salt is used during the winter.

The government investigation is focusing on new corrosion complaints. NHTSA said it had received 346 complaints of corrosion and breaking of the front subframe, which carries the engine, transaxle, steering rack and front suspension. The agency said it had received reports of three crashes and one injury.

Most of the complaints have been logged in the cold weather states and some owners have cited problems on the right side of the vehicle with the routing of the air-conditioning lines. Ford and several owners noted that condensation had been observed dripping from the front subframe.