One of two pickup trucks struck by a runaway trailer in Batavia, OH, in January 2014.

Already under fire for failing to catch a series of deadly defects – including the flawed General Motors ignition switch blamed for over 100 deaths – federal regulators may have missed yet another fatal flaw.

This one involves a potentially defective hitch used on as many as 6,000 semi-trucks plying U.S. highways. Produced by an Alabama supplier, the part has been linked to a case involving a runaway trailer that killed two people on a snowy highway in Batavia, Ohio early last year.

Though the manufacturer had issued several service bulletins, and users had filed several complaints – at least one involving a crash – the National Highway Traffic Safety Administration declined to open a formal investigation until last month.

That was 17 months after a 12-ton semi-trailer came loose while being pulled up U.S. 50, a treacherous stretch of road near Cincinnati known to locals as the “Devil’s Backbone.” According to the Associated Press, the trailer struck the side of a pickup and then slammed head-on into another. The drivers of both those vehicles were killed.

(Feds slam Fiat Chrysler for lax safety efforts. Click Here for the story.)

During a police investigation, the truck’s driver, Michael Simpson, acknowledged having problems hooking up the trailer, though he insisted he finally got the hitch to work and then drove a short distance to ensure it was locked into place.

The Fontaine Ultra LT hitch.

But the manufacturer of the hitch, Fontaine Fifth Wheel of Trussville, Alabama, the incident was the result of the driver’s error, a position Ohio State Police investigators ultimately echoed as they charged 62-year-old Simpson with vehicular homicide. He got probation, rather than jail time, and saw his truck license temporarily suspended.

That led the National Highway Traffic Safety Administration to let the matter go –until Fontaine notified the agency of its own concerns about the Ultra LT hitches.

Earlier this year, the company said it wanted to replace 6,000 Ultra LT hitches, though it claimed that was due to “non-safety” issues.

Now, an agency spokesman told the AP, NHTSA wants to “determine if the service bulletin, the fatal crash and the plan to replace all of the fifth wheels (hitches) have a common safety-related root cause.”

Further investigation has revealed that there has been a higher rate of unexpected truck-trailer separations than might normally be expected with the Fontaine hitches.

The firm, which is owned by Berkshire Hathaway, Warren Buffett’s big investment group, is said to be cooperating with the investigation. Fontaine must turn over documents on the Ohio crash and other records by July 24.

It isn’t clear how long NHTSA will take to complete the investigation.

But the federal agency will clearly be feeling pressure to get things right. New Administrator Mark Rosekind took intense fire during a Senate hearing last month called in response to a new audit of NHTSA’s record.

(For the full story on the Senate hearing, Click Here.)

Outlining a series of shortcomings, the 42-page report by the Transportation Department’s Office of Inspector General declared that, “Collectively, these weaknesses have resulted in significant safety concerns being overlooked.”

NHTSA has been faulted for failing to uncover the ignition switch debacle that led to the recall of 2.6 million General Motors vehicles last year, a problem now linked to at least 119 deaths by a victims’ compensation fund GM set up. The federal safety agency has also been criticized for taking so long to step in on the Takata airbag recall now blamed for the recall of more than 30 million vehicles and eight deaths worldwide.

(After long silence, Takata CEO apologizes for airbag problems. Click Here for more.)