Investigators in Texas will try to determine what critical mistakes were made in response to the deadly shooting at a school in Uvalde, including the question of why nearly 20 police officers remained in front of the classroom while children shouted for help, Reuters reports.
At the heart of the Texas Department of Public Safety investigation is also the question of why officers waited in the hallway for nearly an hour before entering and killing the attacker.
With more calls for an independent investigation and growing criticism of the police response, Houston and Dallas police arrived in Uvalde to assist local forces.
Meanwhile investigators are still searching for the motive of the attack. Salvador Ramos, an 18-year-old high school student who dropped out of high school, had no criminal record and had no history of mental illness.

Emergency calls
At least two children have called for help from several neighboring classes after Ramos entered the school Tuesday with an AR-15 semi-automatic rifle, Texas Public Safety Director Steven McCraw said earlier this week.
"He is in room 112," said the girl, who called for help at 12:03. The same girl asked the ambulance operator to "send the police immediately" at 12:43 and four minutes later.
At 12:51 p.m., almost 45 minutes after her call, the tactical team stormed in and broke the siege of Robb Elementary School.
A total of at least eight ambulance calls were made from the school as police waited outside. Two children who cried for help survived.
McCraw stressed that the school district police chief had misdiagnosed that Ramos had been barricaded at the school and that the children were no longer in danger, thus allowing police to prepare for the attack.
"It was a wrong decision and point," McCraw noted, acknowledging that standard protocols require police to immediately confront an active school attacker, rather than wait for help.

"Lack of clear instructions"
Tactical border patrol agents at the scene were frustrated by the lack of clear instructions from commanders, believing this delayed efforts to stop the attack, a police source said.
The source suggests that when the tactical agents entered the school classroom after the ballistic shield, the attacker came out of the closet and fired at them.
"I hate to believe that some of these children could have been rescued if the police had helped them immediately after they were shot, not an hour after they bled," said another police source.
Official police response reports do not provide clear answers. Texas Gov. Greg Abbott, a day after the assassination, praised police courage and said Friday he was "angry" because investigators had misled him about the information.
Texas Democrat Joaquin Castro called for an independent FBI investigation and visited the school Friday, telling Reuters he was deeply concerned about the conflicting information that emerged.