WASHINGTON — Toyota faced mounting pressure Thursday as the government opened a probe of brake problems with the Prius, a crown jewel of its lineup. The beleaguered automaker said it was "too soon" to decide whether to add the hybrid to the millions of cars it has recalled.

Some owners of the 2010 Prius have reported their brakes do not always engage immediately when they press the brake pedal or that the brakes have an inconsistent feel. The National Highway Traffic Safety Administration said it would assess the scope of the problem and the safety risk to about 37,000 cars that could be affected.

The investigation comes as safety questions surround Toyota, which has already issued broad recalls for millions of its best-selling vehicles, including the Corolla and Camry, because of gas pedals that can become stuck.

U.S. officials have blessed Toyota's solution to that problem, a small piece of steel designed to eliminate excess friction in the pedal mechanism, but have criticized Toyota for being too slow in responding to customer complaints.

Asked whether Toyota would recall the 2010 Prius, spokesman Brian Lyons said: "It's too soon to call at this point. We will, of course, fully cooperate with NHTSA in that investigation."

Lyons also said Toyota is checking other hybrid models in its lineup to see if they have the same braking system as the 2010 model, but said he was so far unaware of any other models being involved.

Congressional investigators expanded their review of Toyota to include the Prius as California Rep. Darrell Issa, the ranking Republican on the House Oversight Committee, asked Toyota for records on its Prius brakes.

The committee plans a hearing next week on Toyota's recalls, the first of two in Congress this month. Issa said he would focus on whether Toyota or NHTSA failed to properly deal with safety complaints or address them quickly enough.

"We think they should have acted more aggressively or quickly," said Issa, who owns four Priuses, none of which fall under the investigation.

Connecticut Attorney General Richard Blumenthal said he and attorneys general in other states might take legal action against Toyota over possible deceptive claims to consumers about the safety of the company's cars.

Toyota said some customers have complained of an inconsistent feel during slow and steady application of brakes on rough or slick roads when the antilock brakes engage. Normally, the brakes grab and release rapidly in reaction to slipping tires.

Paul Nolasco, a Toyota spokesman in Japan, said the time lag drivers feel before brakes kick in stems from the two systems in a gas-electric hybrid — the gas-engine and the electric motor. The brakes work if the driver keeps pushing the pedal, he said.