Hours after the shooter opened fire, U.S. Rep. Michael McCaul of Texas identified the suspected gunman as Spc. Ivan Lopez. However, officials did not say what might have motivated him to kill fellow soldiers and then himself.

"We do know that this soldier had behavioral health and mental health issues and was being treated for that," said Lt. Gen. Mark Milley, head of the Army's III Corps at the Texas post.

Lopez was being evaluated for post-traumatic stress disorder, but had not yet been...