Texas authorities say a teacher closed a propped-open door before the attack

A back door at Robb Elementary School, where a gunman entered through to get into a classroom in last week's shooting, is seen in Uvalde, Texas, on Monday. (Jae C. Hong/AP)

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.