The development raises questions about whether Toyota's repairs will bring an end to the cases of wild, uncontrolled acceleration or if there may be electronic causes behind the complaints that have dogged the automaker.
Although the allegations were unverified by the agency, the National Highway Traffic Safety Administration said Wednesday it was contacting people who have complained about acceleration problems even after repairs were done under two large recalls. The agency wants to hear from others who have had similar troubles, it said.
"If Toyota owners are still experiencing sudden acceleration incidents after taking their cars to the dealership, we want to know about it," agency administrator David Strickland said in a statement.
The new complaints raised eyebrows in Congress, which has held three hearings on the recalls in the past week and is investigating Toyota's safety problems.
"I am deeply concerned that NHTSA has received this many reports of possible sudden unanticipated acceleration even after these vehicles have received Toyota's recommended fix," said Rep. Bruce Braley, D-Iowa, who serves on one of the committees investigating Toyota.
"It's critical that we get to the bottom of this problem as quickly as possible."
Toyota spokesman Brian Lyons said Wednesday the company was investigating the complaints, though it remains confident in its recall fixes. Teams of engineers are being mobilized to check into the complaints, he said.
Toyota has recalled more than 8 million vehicles worldwide since October to fix floor mats that can snag gas pedals or faulty gas pedal assemblies that can stick.
NHTSA says 52 people have been killed in crashes linked to Toyota's acceleration problems. Toyota has blamed mechanical causes or drivers pressing the wrong pedal. However, some question whether the electronic throttle system or a software glitch may be at fault, rather than a mechanical issue involving pedals. Toyota says it is looking into electronics as a possible cause.
In the meantime, complaints keep coming in from drivers who say the fix has not solved the problem, including at least 15 filed with NHTSA in the last two weeks, according to an AP analysis of the agency database.
The owner of a 2009 Camry, Stewart Stogel, of Mount Vernon, N.Y., told the AP he narrowly missed driving over an embankment and hitting a wall when the mid-size sedan accelerated on its own last Saturday - five days after being serviced as part of the recall.
Stogel, who has not filed a complaint with NHTSA, said the incident occurred near his home after a dealership trimmed the gas pedal and installed new brake control software as part of the floor mat recall.
"At first the brakes didn't engage at all," said the 49-year-old freelance journalist. "Just as I approached Terrace Avenue, the wheels were able to get some traction, and all of the sudden the engine did disengage."
Stogel said the car had accelerated two previous times before the recall fix, and both times he took it to dealerships to be checked. In one case it was inspected by a Toyota corporate technician who could find nothing wrong, he said.
After the latest incident, Stogel called his dealer, who told him to return with the car so Toyota engineers could inspect it. He did so on Wednesday and was given a loaner car. He also left a message with Toyota Motor Sales U.S.A. President Jim Lentz.
More on Toyota's troubles:
No Fix? Trouble with Some Repaired Toyotas
Senate Committee Members' Toyota Links
NYT: Troubles Predate Recalled Toyotas
Poll: 49% Say Toyota Hiding Something
More Toyotas Need Oil Hose Fix
Toyota Accused of Hiding Design Evidence
Carolyn Kimbrell, 59, a retired office assistant in Whitesville, Ky., said her 2006 Toyota Avalon accelerated last weekend as she was returning with her 9-year-old granddaughter from a trip to the mall. The incident occurred a week after her dealer inserted a metal piece into the gas pedal mechanism on Feb. 20 to eliminate the friction blamed for the pedal problems.
The dealer said her car wasn't covered by the floor mat recall, but agreed to do that fix after she reported the latest incident, she said.
Now she wonders if the company's remedies will solve the problem. "It just scares you," Kimbrell said.
The recalls have hurt Toyota's safety and quality reputation and brought death and injury lawsuits. Federal prosecutors in New York are conducting a criminal investigation into the recalls and the Securities and Exchange Commission is probing what the automaker told investors.
Electronic causes have come under scrutiny because until now, Toyota and the government have done little to examine them. Most modern cars are essentially computers on wheels, with systems of sensors, wires and computer chips controlling the throttle, brakes, transmissions and other functions.
Software glitches can develop in the computers, electrical interference can cause sensors to send the wrong signals and there can be electrical shorts. Sometimes two or three glitches can happen at once.
Re-creating electronic problems is extremely difficult because often they leave no trace.
The latest complaints are further evidence that the automaker's repairs don't cover all possible causes, said Sean Kane, president of Safety Research and Strategies Inc., a Massachusetts-based car safety investigation and advocacy group who has studied Toyota.
"Anybody who has reviewed the complaints and reviewed what's going on here has to reach a conclusion that there's more going on than sticky pedals, floor mats and drivers," said Kane, whose firm has done research for trial lawyers suing Toyota.
Kane believes the problem has multiple electronic roots, perhaps in cruise-control mechanisms.
So far NHTSA, which has the power to force an automaker to recall vehicles, has gone along with the mechanical causes pinpointed by Toyota.
"What we need this agency to do is really question Toyota, look at the issues in a more thorough way," Kane said. "In the past they were quick to dismiss electronics."