Tens of thousands of truckers who flunk drug tests may still be driving big rigs in violation of federal regulations, a new government investigation has found.

In a 74-page report scheduled for release today, the Government Accountability Office described a flawed oversight system that allows truckers to fail a drug test and yet move on to driving for another company.

Fewer than half of the estimated 85,000 truck drivers who test positive in random drug tests each year are believed to complete the required treatment and follow-up testing to return to their jobs, the GAO said. The true number of truckers who fail drug tests and continue to drive could be much higher, said an investigator with the House Transportation and Infrastructure Committee, which commissioned the GAO study.

"It raises extraordinary concerns," Rep. Peter DeFazio, D-Ore., chairman of the House Highways and Transit Subcommittee, said of the report. "We have a system of drug testing that is totally inadequate. You can easily beat the system."

The GAO report confirms problems documented in a 2006 Dallas Morning News investigation of the trucking industry, including spotty background checks, drug use by truckers and inconsistent oversight by state and federal authorities.

In 2006, 4,995 peopled were killed nationwide and 106,000 injured in crashes involving large trucks, the report noted. Statewide, about 500 people are killed each year in crashes involving large trucks.

Although mechanical problems, speeding and driver fatigue are the most frequent factors in fatal accidents involving big rigs, studies have also found that drugs or alcohol substantially increase the risk of accidents. The trucking industry blames passenger cars for causing the majority of accidents.

The GAO report found that some trucking companies don't bother to conduct the required pre-employment and random drug tests and "have limited incentives" to do so. Only about 2 percent of all trucking companies undergo checks each year by state agencies and the Federal Motor Carrier Safety Administration, which regulates interstate traffic, the GAO found.

In addition, dishonest truckers can beat the testing system by using false IDs and chemicals to alter their urine for drug tests, the report said. If caught, they easily move on to other trucking companies, which the GAO described as "job-hopping."

"If you fail both at beating the test and job-hopping, you state-hop," Mr. DeFazio said. "If you fail a test in Texas, you can go to California, and the states aren't talking to each other. There are so many loopholes that need to be plugged."

Among the report's recommendations is the creation of a national database of truckers who fail drug tests. The American Trucking Associations, which represents the nation's largest carriers, said it would support a national drug and alcohol test database administered by trucking companies.

The exact number of drivers who return to work without completing the required treatment is unknown, the GAO said. But a director of the Substance Abuse Program Administrators Association "conservatively estimates that less than half of commercial driver's license holders who test positive or refuse to test successfully complete the return-to-duty process before returning to their jobs," the GAO report said.

The report concluded that drug use could be significantly higher among truck drivers than what the random test data indicates because not all companies actually test, urinalysis can be unreliable and results can be altered.

For example, GAO investigators who posed as truckers appearing for drug tests weren't required to empty their pant pockets at 10 of 24 sites. The requirement is designed to prevent a driver from using drug-concealing agents or substituting clean urine samples.

Another hole in the drug testing system is the self-regulation required of tens of thousands of owner-operators, the GAO found. These self-employed drivers are "unlikely to remove themselves from safety-sensitive duty in the event of a positive test," the report concluded.

Truckers who test positive or refuse to test are supposed to undergo education, treatment and re-testing before being allowed to return to the road. But the GAO report found that didn't always happen because of a lack of oversight by companies and authorities.

The report did not single out any drivers or companies by name.

In addition to a national database of drug-testing information, the GAO recommended that the Transportation Department speed up efforts to improve safety audits of trucking companies. It also recommended that Congress provide the Federal Motor Carrier Safety Administrationwith "additional authority over entities involved in the drug testing process" and encourage or require states to suspend the commercial driver's licenses of truckers who fail or refuse to take a drug test.

Seven states, including Texas, maintain a database of failed drug tests that trucking companies can consult, the report said. It characterized Texas as having "low reporting from carriers." Texas-based employers are required only to report the drug test results of truckers holding Texas commercial driver's licenses, said DPS spokeswoman Tela Mange. There are 13,279 entries in the Texas database, she said.

•Audit company drug-testing programs to ensure compliance with federal regulations.

•Improve oversight of drug-testing collection facilities and verify trucker identification to prevent use of fake IDs.

•Adopt a federal ban on substances used to beat drug tests.

•Test truckers for a broader range of illegal drugs and some prescription drugs. Currently, truckers are tested only for a handful of the most common illegal drugs.

•Create a national database of truckers who have failed or refused a drug test.

•Create a procedure for suspending the commercial driver's license of truckers who fail drug tests.