At least 15 Toyota drivers have complained to United States safety officials that their cars sped up by themselves even after being repaired under recalls for sticky gas pedals or floor mat problems, according to an Associated Press analysis.

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 auto maker.

Although the allegations were unverified by the agency, the National Highway Traffic Safety Administration said on Wednesday it was contacting people who have complained about acceleration problems even after repairs under two large recalls. The agency wants to hear from others who have had similar troubles.

"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.

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 Democratic Representative Bruce Braley, 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 on Wednesday that the company was investigating the complaints, although 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.