Emergency plans for the Astroworld festival, at which eight people died and hundreds were injured, did not include protocols for dangerous crowd surges.
People were crushed as crowds rushed forward to see headliner Travis Scott at the event in Texas. The surge left a nine-year-old boy in a coma, his family said.
Houston’s police and fire departments played a key role in safety at the sold-out show, which attracted 50,000 people.
But firefighters did not have a presence inside the festival and were not given radios to communicate directly with organisers, the union head of the Houston Fire Department said on Tuesday.
More than 20 lawsuits have already been filed. They accuse organisers of failing to take simple crowd-control steps or staff the event properly.
Bernon Blount said his grandson, Ezra, was in a medically induced coma on Tuesday at a Houston hospital and that the boy’s heart, lungs and brain were injured in the melee.
“My son, once he had passed out from the pressure being applied to him during the concert, he passed out and Ezra fell into the crowd,” Mr Blount told AP. “When my son awakened, Ezra wasn’t there.”
A 56-page event operations plan for the Astroworld music festival included protocols for dangerous scenarios including a gunman, bomb or terrorist threats and severe weather.
But it did not include information on what to do in the event of a crowd surge.
“In any situation where large groups of people are gathering, there is the potential for a civil disturbance/riot that can present a grave risk to the safety and security of employees and guests,” the plan said.
“The key in properly dealing with this type of scenario is proper management of the crowd from the minute the doors open. Crowd management techniques will be employed to identify potentially dangerous crowd behaviour in its early stages in an effort to prevent a civil disturbance/riot.”
Experts say crowd surge deaths happen because people are packed into a space so tightly that they are being squeezed and cannot get oxygen. Injuries or deaths do not usually take place as a result of people being trampled.
Authorities have said part of their investigation will include reviewing whether the concert promoter and others behind the festival adhered to the plans submitted.
Rapper to cover funeral costs
Mr Scott, who founded the Astroworld festival, said he would cover funeral costs for the victims. The dead ranged in age from 14 to 27 and came from Texas, Illinois and Washington state, according to Harris County authorities. They included secondary school pupils, an aspiring border patrol agent and a computer science student.
There is a long history of similar catastrophes at concerts, as well as sporting and religious events. In 1979, 11 people were killed as thousands of fans tried to get into Cincinnati’s Riverfront Coliseum to see a concert by The Who. Other crowd catastrophes include the deaths of 97 people at a football match in Hillsborough Stadium in 1989 in Sheffield, England.