Federal safety regulators have closed an investigation into unintended acceleration in Toyota Tacoma pickup trucks, saying their probe couldn't find a cause behind more than 400 complaints.The National Highway Traffic Safety Administration last Wednesday 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 had argued 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.