The National Highway Traffic Safety Administration said Thursday in Washington it is opening a formal investigation into some new Toyota Prius Hybrids to look into allegations of momentary loss of braking capability while traveling over an uneven road surface, pothole or bump.

NHTSA said it is investigating about 37,000 2010 Prius models sold in the United States. Toyota Motor Corp. in Japan acknowledged that the Prius had a "design flaw" and has made changes to the braking system late last month. But it hasn't said what it will do for current 2010 models, and it hasn't ruled out recall.

Toyota spokeswoman Martha Voss said the company was aware of the investigation.

NHTSA's Office of Defects Investigation has received 124 reports from consumers, including four reports alleging that crashes occurred. Two of those resulted in two minor injuries, NHTSA said. Investigators have spoken with consumers and conducted pre-investigatory field work.

Before a recall could be order, NHTSA would have to determine there was enough evidence to first upgrade the matter to an engineering analysis.

In Japan, at least 77 complaints on Prius brake systems have been received. Japanese media reported that Toyota is now reviewing the braking systems on all of its hybrid models.

"Safety is our top priority," Detroit News quoted Transportation Secretary Ray LaHood as saying. "That is why in recent weeks, NHTSA has also issued a consumer advisory on the recall of several models of Toyota vehicles and the Pontiac Vibe involving pedal entrapment and sticky accelerator pedals. We will continue to monitor these issues closely."

The issue has gotten more attention as owners -- including Apple co-founder Steve Wozniak -- have raised questions about the Prius.

Late Wednesday, LaHood spoke with Toyota president and chief executive Akio Toyoda, who reassured him that Toyota takes U.S. safety concerns seriously and puts safety at the top of the company's priorities, LaHood said in a statement.

Of the 100 investigations NHTSA opens every year, there are currently 40 open defect investigations, three of which involve Toyota. NHTSA has the most active defect investigation program in the world, opening or closing an investigation almost every week.

The latest woes with the Prius come after two massive recalls by Toyota in recent months in connection with sudden unintended acceleration concerns. Toyota said Thursday the recalls could cost the company 2 billion dollars in lost sales and costs by the end of March.

Toyota has recalled 5.4 million vehicles in the United States over worries of pedal entrapment in floor mats and another 2.3 million vehicles over sticky pedal concerns.

Toyota dealers have begun fixing the sticky pedals in the United States. The recalls forced Toyota to stop selling eight popular models in the United States and shutter six North American assembly plants for a week.

U.S. Congress is set to hold at least three hearings now, with the first planned for Feb. 10 before the House Oversight and Government Reform Committee.