An exterior door at Robb Elementary School did not lock when it was closed by a teacher shortly before a gunman used it to get inside and kill 19 students and two teachers, leaving investigators searching to determine why, state police said Tuesday.

State police initially said a teacher had propped the door open shortly before Salvador Ramos, 18, entered the school in Uvalde, Texas, on May 24.

They have now determined that the teacher, who has not been identified, propped the door open with a rock, but then removed the rock and closed the door when she realized there was a shooter on campus, s...