Safety regulators opened a formal investigation into consumer complaints about braking on 2010 Toyota Prius hybrids, the latest jolt to the automaker already reeling from massive recalls and intensified safety scrutiny.

Prius is the world's best-selling hybrid, and U.S. consumers have clamored for the gasoline-electric vehicle as tastes gravitate away from gas guzzlers to cars and trucks with better fuel efficiency.

Toyota Motor Corp shares, which fell 6 percent on the New York Stock Exchange on Wednesday, were off another 2.3 percent in morning trading on Thursday. They closed off 35 percent in Tokyo.

The Transportation Department's National Highway Traffic Safety Administration (NHTSA) said it had received 124 complaints about momentary Prius braking problems after motorists rolled over bumps or potholes.

Motorists allege four crashes and two minor injuries were caused by the problems. The NHTSA investigation covers 37,000 vehicles.

Some consumers report a brief lag in braking or a brief surge while braking, according to a copy of the NHTSA report approving the so-called preliminary evaluation.

That action is the first step in the formal NHTSA investigative process, after reviewing complaints and interviewing motorists. It is unclear whether the matter will proceed to a recall.

Toyota officials said they would fully cooperate with the investigation.

The automaker's engineers in Japan said earlier that they had fixed a software problem related to anti-lock brakes on the new Prius model.

Late on Wednesday, Transportation Secretary Ray LaHood spoke with Toyota President Akio Toyoda, who assured him that Toyota takes U.S. safety concerns seriously, a Transportation Department statement said.

Separately, NHTSA is pressing ahead with a new investigation of complaints about Toyota electronic throttle systems to see if they are in any way related to unintended acceleration.

No formal investigation has been launched in that matter, a long-standing issue that has been investigated numerous times without a defect finding. Safety advocates and some in Congress have questioned whether NHTSA was aggressive enough in those cases.

LaHood said on Wednesday that new complaints have been received.

Toyota says it has found no evidence of accelerator-related safety problems beyond those covered in two recent recalls covering 5.5 million vehicles in the United States and 2.6 million more worldwide.

Two congressional committees are investigating Toyota safety issues and NHTSA's handling of them, with hearings scheduled for February 10 and February 25.

On Thursday, the House Oversight and Government Reform Committee released a letter sent Wednesday to Toyota seeking information received since May about consumer complaints, warranty reports, and accidents relating to alleged Prius braking and cruise control issues.

The panel, which also sought information from NHTSA, also wants any internal Toyota analyses that may have been performed, as well as any service bulletins to dealers or notices to consumers, according to letters sent by Representative Darrell Issa, the committee's top Republican.

(Reporting by John Crawley; Additional reporting by Soyoung Kim in Detroit; editing by Gerald E. McCormick and John Wallace)