(Bloomberg) -- Synthetic drug use by U.S. truckers is a growingand deadly problem, U.S. safety investigators said Tuesday, inconcluding that a Texas trucker was “likely” impaired by amarijuana-like substance when he hit and killed four members of acollege women’s softball team last year in Oklahoma.

At a hearing Tuesday, the four-member NationalTransportation Safety Board rejected the driver’s claim that hewas distracted at the time of the crash because he was reaching fora soda. The 2014 crash prompted the board to urge new testing andprocedures to protect the public from truck drivers impaired bysynthetic drugs.

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Board members said they don’t know the extent of the problem,which may involve thousands of synthetic drugs that elude detectionin most testing for substances like marijuana, cocaine andalcohol.

“All indications are that it’s a growing problem, and we’reseeing it more publicly available,” said NTSB Chairman ChristopherHart in an interview after a hearing in Washington. “This is afresh and scary and growing problem. We need to get some controlsaround these synthetic drugs. It’s shocking to me that if I’m atruck driver, I can buy this in a truck stop.”

Cannabinoids targeted

The safety board recommended new standards and testing todetermine the extent of synthetic drug use, particularlycannabinoids, among commercial truck drivers, and it urged theindustry to take new steps to detect such abuses.

Four students from Northern Central Texas College died on onSept. 26, 2014, near Davis, Oklahoma, when their bus was hit by atruck driven by Russell Staley, who had a history of usingsynthetic drugs. He drove 1,100 feet across a grassy median, neverslowing down or steering before striking the bus. He then droveanother 300 feet into nearby woods.

Investigators said he was unaware that he had even hit the busas his truck continued on into the trees.

After the crash, investigators found a silver pipe with burntresidue of 5-fluoro-AMB, a synthetic cannabinoid, but later testingcould not confirm or rule out its presence in Staley’s blood,according to the NTSB. Investigators ruled out other factors,including fatigue, saying Staley’s failure to respond to drivingoff the roadway for 11 seconds was likely caused by syntheticdrugs.

Seizure-like symptoms

Staley needed help for his dependency on synthetic cannabinoids,also known as K2, and had seizure-like symptoms, his wife told hisdoctor 13 months before the crash, according to records released bythe NTSB. Notes from Staley’s licensed professional counselor inAugust 2013 also showed he said he “had been using designer drugsat work” and needed help with stress, anger, low self-esteem, guiltand depression.

Staley was charged with four counts of manslaughter and facestrial next year.

The injuries to the 15 people in the bus were worse because theyweren’t wearing seat belts, and their vehicle wasn’t strong enoughto withstand the crash, the board said. The softball coach whowas driving also didn’t require the students wear seat belts, andthe college had no policy in place to ensure that he do so, theNTSB added.

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After the crash, investigators sent samples of Staley’sblood to three labs, including two that could test for the presenceof synthetic cannabinoids, but the results were inconclusive. Theyalso could only find a one in 38 chance that the DNA on the pipefound in Staley’s truck was his.

Staley had worked for Quickway Transportation Inc. for only afew months when the accident occurred. He was 47 miles north of theOklahoma-Texas line at 9:05 p.m. that night when the road curvedslightly to the right, and his rig did not.

Four killed

The accident killed Jaiden Pelton, 19, of Telephone, Texas;Brooke Deckard, 20, of Blue Ridge, Texas; Meagan Richardson, 19, ofWylie, Texas; and Katelynn Woodlee, 18, of Dodd City, Texas. Theteam was returning home from a scrimmage at Southern NazareneUniversity in Oklahoma.

The NTSB released 2,000 pages of documents late Monday,including a transcript of a whistle-blower call to the board from asupervisor who worked for Staley’s previous employer. Staley hadbeen missing work, showing up late and not performing well, thesupervisor told a board investigator.

Staley told the supervisor he was smoking synthetic marijuana,which he referred to as spice. Staley told him that he had passedout in a park and didn’t know how long he had been there. When thesupervisor told his boss about Staley’s problem, “it fell on deafears,” and the company neither took him off the road nor got himassistance, according to the report.

“I told him, I said this guy’s going to kill somebody someday,you watch,” the supervisor said he told his boss. “And I’ll bedamned, it happened.”

--With assistance from Margaret Newkirk.

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