BATAVIA, Ohio (AP) _ On a twisty, snow-lined hill that the locals call “Devil’s Backbone,” a 12-ton semi-trailer came loose from its tractor and plowed into an oncoming line of pre-dawn commuters.

At 40 miles per hour, the trailer struck the side of one pickup truck and careened head-on into another, killing the drivers of both vehicles.

At first the Jan. 24, 2014, crash on U.S. 50 in Cincinnati’s eastern suburbs drew only the attention of Ohio authorities, who faulted the semi driver for not properly inspecting the hitch that holds the trailer to the tractor. But 17 months later, the National Highway Traffic Safety Administration has raised the possibility that the hitch was defective.

On June 9, the agency began investigating a potentially high rate of trailer separations for the hitch involved in the crash _ the “Ultra LT” made by Fontaine Fifth Wheel of Trussville, Alabama. Fontaine says it is cooperating with the probe.

The Ultra LT could be in use on as many as 6,000 semis across the nation.

Given the nearly 1 1/2-year gap between the crash and the investigation, the safety agency could face renewed criticism for failing to analyze its own data to uncover a safety problem _ the same failure that delayed recalls of defective General Motors ignition switches and faulty Takata air bags.

Although the agency’s new administrator says reforms are underway, one frequent critic of NHTSA sees remnants of an old problem. “(The Fontaine case) sounds like an artifact of what we used to see from the agency over time, an inability to connect the dots,” says Sean Kane, president of Safety Research and Strategies.

Kane says the agency needs to move quickly given the number of semis using the Ultra LT hitches.

The agency says it acted properly, opening the investigation after Fontaine said it wanted to replace all 6,000 hitches for unspecified “non-safety” reasons.

The agency investigated the hitches once before, in 2011, after Fontaine issued a service bulletin. NHTSA found 12 complaints about the hitches, plus one crash with no injuries. Truck companies Freightliner, Kenworth, Volvo and Mack recalled 2,400 tractors to replace a bar that locks a pin from the trailers to the hitch.

NHTSA said Fontaine made a design change at that time to prevent the problem from occurring in future products.

In 2012, Fontaine revealed another problem with the Ultra LTs in a second service bulletin, but the agency decided the issue didn’t warrant an investigation.

As required by law, Fontaine notified NHTSA of the Cincinnati-area crash in August of last year, as did the truck’s maker, Navistar. Fontaine subsequently submitted reports that blamed the crash on an improper hookup by the driver, so the agency didn’t pursue the matter further.

Once Fontaine said it wanted to replace the hitches, NHTSA decided to act. Agency spokesman Gordon Trowbridge says the investigation will “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.”

Fontaine has until July 24 to turn over communications on the Ohio crash and other information.

Steve Mann, vice president of engineering for Fontaine, which is part of Warren Buffett’s Berkshire Hathaway Inc., says in a statement that the company analyzes data to help ensure “safety and reliability.” NHTSA said in documents that Fontaine believes operator error caused the Devil’s Backbone crash.

Before he drove on the hill that frigid January morning, Michael Simpson tried to hook his grocery trailer to a tractor at a truck yard north of Cincinnati. Three times, it didn’t latch, he told police. It locked on the fourth try, and Simpson drove a short distance to make sure. He checked again while en route to an IGA store, saying he was “concerned.”

Then, about 6:30 a.m., as he was climbing Devil’s Backbone, the trailer came loose. It tore across the side of Michael Brown’s Chevy Silverado, killing the 43-year-old father of three. Then it hit Shawn Wilson’s Dodge Ram head-on, killing the 39-year-old father who had one daughter.

“It was a terrible accident. Just so sad,” said Kathy Sabo, who saw it happen while driving to work and tried to help victims.

The Ohio State Highway Patrol investigated, finding that the hitch didn’t lock due to a combination of the minus 4 degree temperature and a buildup of frozen grease on the pin and receiver.

Days after the crash, tests showed the hitch worked just fine. Sgt. Charlie Scales, a reconstruction expert, says Simpson would have spotted the loose connection if he had properly inspected the hitch.

Simpson, now 62, was convicted earlier this year of vehicular manslaughter, a misdemeanor. His commercial driver’s license was suspended for 90 days and he got a year’s probation. His attorney in a lawsuit stemming from the crash wouldn’t comment.

Scales says loose trailers and a lack of proper visual inspections by truckers are common. But in most cases, an unhitched trailer stops safely and is re-attached to the truck, he says.