WASHINGTON, D.C. --- Federal safety regulators have closed an investigation into unintended acceleration in Toyota Tacoma pickup trucks, the Detroit Free Press reported.

The National Highway Traffic Safety Administration typically closes such probes when they don't reveal a clear fault in vehicles. Over the past eight years, the agency has closed at least six other investigations into reports of unexpected or uncontrolled acceleration in vehicles without finding evidence of defects, and has said in the past that driver error is often the cause, the newspaper reported.

NHTSA last week denied a petition by a Tacoma owner who said his truck suffered two instances of sudden acceleration in January and noted 32 similar complaints to NHTSA. Toyota Motor Corp. told the agency earlier this year that 431 customers had complained of such events, including 51 crashes and 12 injuries.

But Toyota also said that many of the complaints were "inspired by publicity," and no flaws had been found in the truck. NHTSA's investigation covered 775,000 Tacomas built between the 2004 and 2008 model year, the Detroit Free Press reported.

"For those vehicles where the throttle control system did not perform as the owner believes it should have, the information suggesting a possible defect related to motor vehicle safety is quite limited," the agency said in its notice. "Additional investigation is unlikely to result in a finding that a defect related to motor vehicle safety exists."

The agency said its investigators interviewed 64 customers who complained to the agency. In many of those cases, NHTSA said the complaints were linked to engine controls that caused the Tacoma's engine to surge slightly, but would produce no or minimal extra power.

In 14 cases, investigators said loose floormats may have caused a problem. In five cases, drivers had stepped on both the accelerator pedal and brake. But in 26 cases, including 13 crashes and four injuries, the agency found no explanation for the complaints.

Toyota told the NHTSA last month that most of the claims to the automaker and the agency dealt with minor engine speed changes unrelated to safety. Toyota added that its own investigation last October of 12 trucks reported to have sudden acceleration problems found no defects, the newspaper reported. No pickup model other than the Tacoma has garnered more than a few complaints of sudden acceleration to the agency in recent years.

The Tacoma uses a drive-by-wire system. Computer controls replace a direct physical connection between the accelerator pedal and the throttle. According to Toyota, the system was designed to report an error in case the accelerator pedal and throttle are mismatched. Toyota has not found error codes in vehicles inspected either by technicians or mechanics at dealerships.