The U.S. Transportation Department opened an investigation Thursday into new problems surfacing with the braking system on the gas-electric hybrid Toyota Prius, specifically the 2010 model.

The National Highway Traffic Safety Administration reported more than 100 complaints of poor brake performance filed against the 2010 Prius, the third-best selling Toyota model introduced last summer. No official recall has been ordered, but Toyota said in a statement that it would fully cooperate with NHTSA's investigation.

The 2010 Prius brakes were not a part of the two major recalls made on Toyota vehicles earlier this month.