WASHINGTON (CNN) -- Federal safety investigators said Tuesday they fear flaws found in Washington's Metro subway system after a deadly crash this summer may endanger other transit systems, and they sent out an urgent recommendation asking that other rail operators check for similar problems.

Investigators are shown at the crash scene in Washington. Nine people were killed in the June 22 wreck.

In letters to federal regulators, the National Transportation Safety Board said "all rail transit operators and railroads should be informed" about system flaws that could cause a track circuit to fail to detect a train.

It was not clear how many rail systems have similar train detection systems. Neither the NTSB nor the Federal Transit Administration had a list of systems that use the "audio frequency track circuits" that are the focus of the probe.

But an FTA spokesman said that because it doesn't know how many operators use the systems, "we are sending today's urgent recommendation to all rail transit operators, and will identify the pertinent operators through a later survey."

Meanwhile, the Federal Railway Administration, which regulates Amtrak and more than a dozen commuter rail systems, said it also will follow the NTSB guidance but believes the number of impacted systems will be "limited."

Nine people were killed and 52 taken to hospitals June 22 when a southbound Metrorail train struck the rear end of stopped train just north of the Fort Totten station. Investigators say an automatic train protection system did not detect the stopped train, so the moving train did not receive a command to slow or stop.

The NTSB said it is continuing its investigation into the precise causes of the crash, but it said the investigation has raised concerns that the track circuit is susceptible to errant signals.

In its letter to regulators, the NTSB said it discovered one circumstance in which an unintended signal path could be created, resulting in a track relay remaining energized even though a stopped train was occupying the circuit.

"After only three months, this complex investigation is far from complete, so we are not ready to determine the probable cause of the [Metro] accident," NTSB Chairwoman Deborah A.P. Hersman said. "However, our findings so far indicate a pressing need to issue these recommendations to immediately address safety glitches we have found that could lead to another tragic accident."

The NTSB sent letters to Metro, the Federal Transit Administration, the Federal Railroad Administration and Alstom Signaling Inc., which acquired General Railway Signal, the manufacturer of some of the equipment.



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