The investigation added 1.4 million cars and SUVs, including 600,000 more Camrys from the 2007 and 2009 model years. The expanded probe also includes some 2007 and 2009 Yaris subcompacts and all 2008 Highland Hybrid SUVs.
So far, Toyota and the government have received 161 complaints of fires involving the vehicles. Nine people have been hurt.
All of the vehicles use the same power switch in the driver's door. The switches can overheat and cause fires. Owners of the Toyotas who smell smoke or feel heat in the doors should call their dealers or take them in for an inspection, Toyota spokesman Brian Lyons said. The company is cooperating with The National Highway Traffic Safety Administration in the investigation, he said.
The investigation began in February and initially involved more than 800,000 Camrys and RAV4 SUVs from the 2007 model year. NHTSA has upgraded the investigation to a so-called engineering analysis, which can lead to a recall.
The Associated Press contributed to this report.