WASHINGTON, April 2 (Reuters) -

The Senate Commerce Committee may subpoena the U.S. Army to

obtain a report detailing why it routinely failed to use a

safety system known as ADS-B on helicopter flights around Reagan

Washington National Airport, its chair said on Wednesday.

An army Black Hawk helicopter did not have the system

operating during a routine training mission when it collided

with an American Airlines ( AAL ) regional jet on January 29

near the airport, killing 67 people.

The army last week refused a request from senators for the

memo.

Committee chair Senator Ted Cruz, a Republican, and Senator

Maria Cantwell, the top Democrat on the panel, had demanded the

army turn over the report by Friday.

Cruz said if the army continued to withhold the document,

the committee planned to issue a subpoena.

"It begs the question, what doesn't the army want Congress

and the American people to know about why it was flying

partially blind?" Cruz said. "This is not acceptable."

The army did not immediately comment Wednesday but said

earlier this week that it would respond directly to the

lawmakers.

ADS-B, or automatic dependent surveillance-broadcast, is an

advanced surveillance technology that transmits an aircraft's

location.

Cruz said if civilians were to die in another collision

between a passenger jet and a Black Hawk helicopter that was not

using the safety system "those deaths will be on the army's

hands."

Civilian airplanes are required to use ADS-B, but the

Federal Aviation Administration in 2019 gave the military an

exemption in rare circumstances. Senators say the military has

rarely if ever used ADS-B in Washington training flights.

The FAA said on Monday it sent notices last week to NASA,

the Justice Department, the Pentagon and Department of Homeland

Security requiring ADS-B use near Reagan National except in

cases such as "active national security missions."