The police response to Tuesday's Texas school massacre was under increasing scrutiny on Friday, a day after authorities acknowledged that more than an hour passed after the gunman entered the building and killed 19 children and two teachers inside their classroom while barricading himself inside.

Even as the shooting - the deadliest U.S. school attack in nearly a decade - intensified the long-standing national debate over gun laws, the National Rifle Association, the country's leading gun rights advocacy group, opened its annual meeting on Friday in Houston. Prominent Republicans, including former President Donald Trump and U.S. Senator Ted Cruz of Texas, were expected to address the convention.

Republican Texas Governor Greg Abbott, who had also been scheduled to speak at the meeting, will instead provide recorded remarks and head to Uvalde, the site of the shooting, to hold an afternoon news conference with other officials.

President Joe Biden, a Democrat who has urged Congress to approve new gun restrictions, will visit Uvalde on Sunday.

Authorities on Thursday provided fresh details about the rampage carried out by 18-year-old Salvador Ramos at Robb Elementary School in Uvalde, a community of 16,000 people about 80 miles (130 km) west of San Antonio.

The new timeline, which differed sharply from some previous official accounts of the shooting, fueled new questions about whether police may have had an opportunity to intervene sooner.

Videos emerged earlier on Thursday showing desperate parents outside the school during the attack, pleading with officers to storm the building, with some fathers having to be restrained.

At a briefing for reporters, Texas Department of Public Safety spokesperson Victor Escalon said preliminary reports that Ramos had encountered a school police officer after crashing his pickup truck near the building were wrong.

Instead, Ramos scaled a fence and entered through an unlocked door around 11:40 a.m. (1640 GMT), 12 minutes after the crash. The door, which is normally locked, may have been left open to accommodate parents who were attending an awards day, according to Miguel Cerrillo, the father of a third-grader at the school.

Two responding officers entered the school four minutes later but took cover after Ramos fired multiple rounds at them, Escalon said.

The shooter then barricaded himself inside the fourth-grade classroom of his victims, mostly 9- and 10-year-olds, for an hour before a U.S. Border Patrol tactical team breached the room and fatally shot him, Escalon said.

The interval before agents stormed in appeared to be at odds with an approach adopted by law enforcement agencies to confront "active shooters" at schools immediately to stop bloodshed.

Asked whether police should have made entry sooner, Escalon answered, "That's a tough question," adding that authorities would offer more information as the investigation proceeded.

Law enforcement officials will brief the media again on Friday afternoon.

The human toll deepened with news that the husband of one of the slain teachers died of a heart attack on Thursday while preparing for his wife's funeral.

Investigators are still seeking a motive. Ramos, a high school dropout, had no criminal record and no history of mental illness. Minutes before the attack, however, he had written an online message saying he was about to "shoot up an elementary school," according to Governor Abbott.

The gunman's father, also named Salvador Ramos, 42, expressed remorse for his son's actions in an interview published Thursday by news site The Daily Beast. The teenager had been living with his grandmother.

"I just want the people to know I'm sorry, man, (for) what my son did," the elder Ramos told the site. "He should've just killed me, you know, instead of doing something like that to someone."