DETROIT (AP) -- A nonprofit auto safety advocacy group is asking Ford to recall 1.35 million Explorer SUVs due to continued complaints of exhaust fumes in the passenger compartments.

The Center for Auto Safety says it found 44 complaints in a government database about fumes and potential carbon monoxide after owners had taken Explorers in for free repairs in a customer service campaign.

The center made its request in a letter to Ford CEO Jim Hackett this week.

The National Highway Traffic Safety Administration has been investigating the problem for two years in police and civilian Explorers from the 2011 through 2017 model years, but it has not reached a conclusion.

Ford says Explorers are safe, owner complaints have decreased and the free service has addressed the exhaust odors.

Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.