Trinity Industries, the highway guardrail maker accused of selling systems that can malfunction during crashes and slice through cars, was found by a jury on Monday to have defrauded the federal government.

The case was brought under the False Claims Act by Joshua Harman, a competitor who discovered that the company made changes in 2005 to its rail head — the flat piece of steel at the front of the system — without telling the Federal Highway Administration, as is required. The company sold the guardrails to state governments that, in turn, received federal reimbursement.

A Texas jury on Monday awarded $175 million that will, under federal law, be tripled to $525 million. The money will be split between the United States Treasury and Mr. Harman, who, although a competitor to Trinity, is considered a whistle-blower. After discovering the design change during litigation in 2011, he filed the case on behalf of the government.

“We are pleased the jury recognized what has been overlooked for three years,” said Nicholas Gravante of Boies, Schiller & Flexner, one of the lawyers representing Mr. Harman.