Alarms have been raised over millions of defective tire valve stems that could pose a safety hazard to motorists, according to an investigation by our affiliate WCVB in Boston, Mass.

Team 5 Investigates reports that the suspect valves are prone to cracking, which can cause a sudden loss of tire pressure. The valve stem is the small inexpensive rubber tube used to inflate a tire.

The family of Robert Monk blames one of the faulty valve stems for the vehicle accident that killed Monk. According to a lawsuit filed by Monk's family against the distributor of the valve stems, a crack in a stem on the tire of Monk's SUV led to a rapid loss of air, which caused the vehicle to flip over.

The faulty valve stems were made at a factory in China and distributed by Dill Air Control Products. Dill says there are as many as 30 million of the Chinese valve stems are in circulation and has warned auto service centers about the problem.

"We don't where they are," said tire safety expert Sean Kane. "We don't how many cars have them. We know there are millions of them here in the United States."

(Click here to watch WCVB's full report.)

While the cracked stems were initially believed to only affect replacement tires, the problem may also threaten tires found on new vehicles. Team 5 Investigates found a 2007 Ford Explorer with valve stems from the Chinese factory, and three of the four stems were cracked.

Without commenting on the Monk lawsuit, Dill says it takes concerns over the valve stems very seriously and has launched an investigation into the problem. Ford has not responded to Team 5 Investigates requests for comment. The National Highway Traffic Safety Administration is also looking into the problem.