HOUSTON — The first images showing armed police waiting in a corridor during last month's school shooting in Uvalde, Texas have emerged.

Police arrived earlier in the attack and with more firepower than previously reported, according to US media who have seen investigative documents.

The images show police holding rifles and riot shields.

Police have been accused of failing to act quickly enough to stop the attack, which left 21 people dead.

Texas police have not publicly commented on the reports, from the Austin American-Statesman and KVUE-TV.

More details of the May 24 shooting at the Robb Elementary School classroom are expected to be presented at a public hearing in the Texas Senate later on Tuesday.

The attack by an 18-year-old local — identified as Salvador Ramos — has led to renewed national debate about gun regulations.

The Austin American-Statesman and KVUE say the documents, the source of which has not been revealed, show that the gunman entered the school at 11:33 local time.

They say 11 police officers arrived within three minutes, and an officer with a ballistic shield was in the building at 11:52.

This happened after school district police chief Peter Arredondo had reported that the gunman had "shot a lot" and that the police only had "pistols", according to the media outlets.

The new information shows Arredondo also tried to speak to the gunman, asking him whether he could hear him, the Austin American-Statesman says.

The police finally breached the door into the classroom where the gunman was at 12:50. He was shot dead by members of the US Border Patrol Tactical Unit.

During the attack, children were frantically calling 911 to report multiple gunshot victims. Worried parents also tried to rush in, as police physically prevented them from entering.

Investigators say that messages from the children were not being relayed to the officers at the scene, who were waiting for more firepower to arrive before confronting the rifle-wielding killer.

Officials said the police wrongly thought that the shooter was contained and held their positions even after further shots were heard from inside the classroom.

The police also waited for a master key to arrive so that the classroom doors could be opened, even though it had not been established that the doors were locked, while tools were also available that could have been used to break them down, the Texas Tribune reported.

The reported delayed response has been the focus of several investigations on both state and federal levels.

On Monday, the parents of the victims and other members of the community called for the resignation of Arredondo at a meeting of the school's board. — BBC