The probe covers about 30,000 of the midsize cars from the 2007 and 2008 model years.
The National Highway Traffic Safety Administration says it has 59 complaints about intermittent loss of power-brake assist. The agency says the problem happens without warning. It causes increased stopping distances and requires more pedal pressure to stop the car.
Two crashes have been reported but no injuries. The agency says 24 incidents happened at speeds of 40 miles per hour or more. It says the number of complaints is increasing, with 55 percent received in the past eight months.
Investigators will try to find the cause and determine if the cars should be recalled.
Toyota spokesman John Hanson said the company is cooperating with NHTSA.
A Camry owner complained to NHTSA in June that the dashboard brake warning lights came on and his brakes failed while approaching a pedestrian crossing. "I did a sharp evasive turn and hit the curb hard in an attempt not to run over pedestrians in the crossing!" the driver wrote. The dashboard lights then disappeared and brakes functioned normally, the owner wrote.
The car was taken to a dealer, who at first found no trouble codes in the Camry's computer. After further testing, the dealer said there was a problem with the brake control computer, the driver wrote. "Toyota is behaving immoral not recalling the faulty parts that in many cases cannot even be diagnosed," the driver wrote.