Follow CBSDFW.COM: Facebook | Twitter

WASHINGTON (CBSDFW.COM/AP) — The Army on Thursday grounded all aircraft in active-duty units across the country for the next five days in order to review safety and training procedures after two deadly helicopter crashes over the past two weeks.

READ MORE: Grapevine Police Seek Armed Robber Who Hit Convenience Store On William D. Tate Avenue

General Robert Abrams, head of U.S. Army Forces Command at Fort Bragg, N.C., ordered the safety stand-down. He said soldiers in all the aviation units at 11 U.S. Army bases will review flight planning, operations standards, aircraft maintenance training and supervisory responsibilities in order to avoid any future accidents.

“We cannot allow tragedy to pass unacknowledged,” Abrams said. “We must do whatever is needed to make certain that our soldiers are training and operating safely.”

READ MORE: 'I Feel Like I'm Doing Something That Actually Matters' Says North Texas Mom Who Became Truck Driver Amid Nationwide Shortage

Two Army pilots were killed when their helicopter crashed near Fort Campbell in rural Tennessee Wednesday evening. And Sgt. 1st Class Toby A. Childers, of Hays, Kansas; Chief Warrant Officer 3 Stephen B. Cooley, of Cantonment, Florida; Sgt. 1st Class Jason M. Smith, of Destrehan, Louisiana; and Chief Warrant Officer 2 Michael F. Tharp, of Katy, Texas, were all killed early last week when their Black Hawk helicopter crashed during a routine training exercise at Fort Hood in Texas.

Unit-level investigations of the crashes are under way but the specific causes of the crashes have not been determined.

MORE NEWS: Tarrant County Prosecutor Chris Taylor Appointed Judge For Texas' 48th District Court

(©2015 CBS Local Media, a division of CBS Radio Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)