At least two girls cowering in their classrooms at Robb Elementary School in Texas called the police emergency number 911 during the shooting, a law enforcement official said.

Gunman Salvador Ramos killed 19 students, mostly nine and ten-year-olds, and two teachers in the attack.

The first call came from a little girl at the Uvalde school who called multiple times on Tuesday.

At 12.03pm local time that day, she identified herself and told authorities what room the shooter was in, said Col Steven McCraw, director of the Texas Department of Public Safety.

"I warn you it's not -" Col McCraw broke off before he recounted the calls. "It's better that I read it than you listen to it."

Here is a 911 emergency call timeline that he outlined at a news conference:

12.03pm - The first call came from a little girl who said she was in Room 112. "She identified herself and whispered, 'He's in Room 112,'" Col McCraw said, without identifying the pupil.

12.10pm - The same girl called back and advised there were multiple people dead.

12.13pm - The girl called again.

12.16pm - She called back and said there were eight to nine students alive.

12.19pm - A 911 call was made from the phone of another child, in Room 111. She hung up when another student told her to hang up, Col McCraw said.

12.21pm - Authorities hear on the 911 call that three shots were fired. (It was not clear which 911 call they were listening to at that time).

12.36pm - A 911 call lasted for 21 seconds

Unspecified time - The initial caller called back and was told to stay on the line and be very quiet. She told 911 "that he shot the door."

12.43pm and 12.47pm - "She asked 911 to please send the police now," Steven McCraw said.

12.46pm - She said that she could hear the police next door.

12.50pm - Shots could be heard on the 911 call, Col McCraw said.

12.51pm - The 911 call is very loud and it sounds like officers are moving children out of the room.

"At that time, the first child that called was outside before the caller cuts off," Col McCraw said.