The National Highway Traffic Safety Administration said the investigation has been upgraded to an "engineering analysis," The Detroit News reported Saturday.
At issue are 22 complaints of rust that makes it difficult or impossible to latch the third-row seat to the floor of the vehicles. About 82,000 vehicles could be affected by the investigation.
The NHTSA said it was looking at 2005-2007 Freestar and Monterey vehicles for comparative reasons. The agency has also asked Nissan Motor Co., for data concerning 2003-2004 minivans to use in comparison with Ford's track record.
The third-row seats in Ford's 2005-2007 vehicles have the same design, regulators said.
Ford said it is cooperating with the investigation. Regulators said they were unaware of any crashes or injuries that resulted from the rust problem.