Washington ? Private laboratories are increasingly being caught falsifying test results for water supplies, petroleum products, underground tanks and soil, hampering the government’s ability to ensure Americans are protected by environmental laws, investigators say.

The fraud has caused millions of people to fill their cars with substandard gasoline that may have violated clean air standards, or to drink water not properly tested for safety, the officials told The Associated Press.

In addition, officials making decisions at hazardous waste cleanup sites have relied on companies that fraudulently tested air, water and soil samples.

“In recent years, what has come to our attention is that outside (nongovernment) labs are oftentimes in bed with the people who hired them, and conspired to commit environmental crime,” said David Uhlmann, chief of the Justice Department’s environmental crimes section.

The EPA’s watchdog against fraud, Inspector General Nikki Tinsley, has called the rise of lab fraud a disturbing trend. “If it was my drinking water I’d consider it very serious,” she said, declining to identify locations affected by the ongoing investigation.

Private laboratories test products that are regulated by anti-pollution laws, and the results allow companies to certify that they’re meeting the requirements of environmental protection laws.

In one instance three years ago, investigators discovered fraudulent test results by contract employees at the Environmental Protection Agency’s lab in Chicago. The head of the laboratory was transferred and the contractor, Lockheed Martin, was suspended from performing tests.

The Justice Department and Environmental Protection Agency have prosecuted dozens of employees and laboratories the past several years for fraudulent testing. Uhlmann, the Justice Department official, said the prosecutions have grown but statistics are not kept on lab fraud cases.

The growing number of cases stretch from New England, where a chemist for municipal water made up test results, to Texas, where the government recently prosecuted the largest tester of underground fuel tanks.

Officials said they aren’t certain whether an increasing number of labs are falsifying tests, or whether more are simply being caught through more aggressive investigations and whistle-blowers.