WASHINGTON (AP) -- The Transportation Department opened an investigation Thursday into brake problems in the 2010 Prius, the latest in a series of safety troubles at Toyota that have confused drivers and strained the Japanese automaker's relationship with U.S. regulators.
Toyota earlier Thursday acknowledged design problems with the brakes in its prized gas-electric hybrid, but said it was still deciding how to inform customers and whether a recall is needed.
The National Highway Traffic Safety Administration, the Transportation Department's safety agency, said it has received 124 reports from consumers about the Prius brakes, including four reports of crashes. The investigation will look into allegations of momentary loss of braking capability while traveling over uneven road surfaces, potholes or bumps.
The U.S. investigation, while preliminary, represents another setback for Toyota, which has been battered with two major recalls in the United States covering millions of vehicles. Those involve gas pedals that can get trapped under floor mats or become stuck on their own and fail to return to the idle position. The safety probes have challenged Toyota's long-standing reputation for building safe, quality vehicles.
The Prius is not part of the gas pedal recalls.
NHTSA said investigators have talked to consumers and conducted pre-investigatory field work. The preliminary evaluation involves about 37,000 vehicles in the United States.
"Safety is our top priority," said Transportation Secretary Ray LaHood. NHTSA said LaHood spoke with Toyota president Akio Toyoda late Wednesday and was assured by the executive that Toyota was taking the safety concerns seriously.
Toyota said in a statement it would fully cooperate with NHTSA's investigation.
The U.S. investigation came after the Japanese government ordered Toyota to investigate brake problems in the Prius, the world's best-selling hybrid. Toyota said it had already corrected problems with the antilock brake system in Prius models sold since late last month, including those shipped overseas.
The new version of the gas-electric Prius hybrid went on sale in the U.S. and Japan in May 2009.
Paul Nolasco, a company spokesman in Japan, said the time lag for brakes kicking in felt by drivers stem from the two systems in a gas-electric hybrid -- the gas-engine and the electric motor.
When the car moves on a bumpy or slippery surface, a driver can feel a pause in the braking when the vehicle switches between the traditional hydraulic brakes and the electronically operated braking system, he said. The brakes work if the driver keeps pushing the pedal, he said.
NHTSA said it opens 100 investigations every year and there are currently 40 open defect investigations, three of which involve Toyota. NHTSA said its defect and compliance investigations have resulted in 524 recalls involving 23.5 million vehicles during the past three years.
AP Business Writer Yuri Kageyama in Tokyo contributed to this report.