According to the Sacramento Bee, a California Department of Transportation technician and his supervisor faked stress results and threw out the original data.
The department announced the firing of Duane Wiles and his supervisor, Brian Liebich, on Monday.
Caltrans said it had reviewed all of Wiles' work dating to 2004 and found he falsified data on three projects: the La Sierra Bridge on State Route 91 in Riverside County, the Braddock Drive undercrossing retaining wall on the 405 Freeway in Culver City and an overhead sign on Interstate 580 in the Bay Area.
"An extensive investigation by both Caltrans and the Federal Highway Administration determined that the falsification was limited to this one employee and to these three structures," Caltrans said in a statement.
"We've gone back and evaluated each of those structures," said Malcom Dougherty, the acting Caltrans director. "One of them was discovered during construction so we addressed that one as we had the opportunity. The other two we've gone back and evaluated they were safe, and they have been deemed safe and adequate."
In all, more than two dozen structures were reviewed, including the California State Route 57 and California State Route 60 high-occupancy vehicle connector in Diamond Bar.
According to the Sacramento Bee, Caltrans officials first learned of possible falsified tests in September 2008, when another department employee spotted inconsistent figures while evaluating Wiles' data for a report.
The Associated Press contributed to this report.