AUSTIN, Texas — The head of the Texas State Police on Tuesday offered a pointed and emphatic rebuke of the police response to a shooting last month at Robb Elementary School in Uvalde, Texas, calling it “an abject failure” that ran counter to decades of training.

In his comments before a special State Senate committee in Austin, Steven McCraw, the director of the Department of Public Safety, provided the most complete public account yet of his agency’s month-old investigation and a forceful argument that officers at the scene could have — and should have — confronted the gunman without delay after arriving. Just minutes after a gunman began shooting children on May 24, he said, the officers at the scene had enough firepower and protective equipment to storm into the classrooms.

“The only thing stopping a hallway of dedicated officers from entering Room 111 and 112 was the on-scene commander,” Mr. McCraw said.

But the commander “decided to put the lives of officers ahead of the lives of children,” he said, delaying the confrontation with the gunman for more than an hour while he “waited for a key that was never needed.”

Most of the victims appear to have been shot in the gunman’s first few minutes in the classroom. But Mr. McCraw’s testimony addressed a central, and painful, question that still hung over the massacre and the delayed police response, one that investigators have attempted to answer through interviews with officers and reviews of video: Were the doors to the classrooms locked, preventing police officers from entering in time to save others?

“I don’t believe, based on the information that we have right now, that that door was ever secured,” Mr. McCraw said of the classroom door that the gunman entered. “The door was unsecured.”

He said classroom doors in the school ordinarily would have been set with a key to lock automatically when closed. But the gunman had been able to enter the classroom, he noted, suggesting that either the door had not been set to lock, or was not fully closed. A teacher had made a request before the shooting that the lock be fixed, he said, adding that the lock was not broken but the so-called strike plate was “dysfunctional,” requiring someone to pull on it to get it closed.

In any case, he said, “There’s no way to lock the door from the inside. And there’s no way for the subject to lock the door from the inside.”

Mr. McCraw focused his blame on the on-scene commander, whom he identified as the chief of the Uvalde school district’s Police Department, Pete Arredondo, who he said was the highest-ranking person at the scene.

The chief has said he did not consider himself in charge, but Mr. McCraw disputed that. “If you’re going to issue commands, if you’re going to direct action,” he said, “you’re the on-scene commander.”

The delayed confrontation with the gunman, Mr. McCraw said, was “antithetical to everything we have learned over the past two decades since the Columbine massacre” in 1999.

Several of the senators reacted with shock and anger. “Every shot is a death,” said Senator Paul Bettencourt, a Republican from the suburbs of Houston. “And yet this incident commander finds every reason to do nothing.”

“I challenge this chief to testify in public,” Mr. Bettencourt said loudly at one point, referring to Chief Arredondo. The chief was also in the State Capitol on Tuesday, testifying before a closed-door hearing of a Texas House investigatory committee. He did not speak to the news media before or after.

A lawyer for Chief Arredondo did not respond to a request for comment, and the chief has said he does not want to discuss the case further until the investigation has been completed.

Mr. McCraw has been the director of the Department of Public Safety since 2009 and oversees both the State Police and the Texas Rangers, the organization conducting the investigation into the Uvalde shooting. A native of El Paso, Mr. McCraw started as a Texas state trooper in the 1970s and later rose through the ranks of the F.B.I. before returning to Texas law enforcement as the state’s homeland security director under Gov. Rick Perry.

His testimony, over more than four hours, was unusually charged because it followed weeks of little to no official updates on the investigation and came after what had been a halting and troubled initial effort by top state officials to provide details about the shooting and the police response.

On Tuesday, Mr. McCraw brought poster boards showing a timeline of the shooting and police response at the school, photos of doors at the school, and two maps depicting how the gunman and police officers entered the school and then the two connected classrooms. He walked between them as he presented investigators’ findings to the assembled state senators. He also had a portion of a classroom door, taken from Robb Elementary, and demonstrated its locking mechanism.

The senators asked direct questions about the response, but also addressed the broader political debates over school safety and gun regulation that have erupted in the wake of the shooting in Uvalde.

“It doesn’t take a gun,” Senator Bob Hall, a Republican from East Texas, said. “This man had enough time to do it with his hands. Or a baseball bat.”

Jon Rosenthal, a Democratic member of the Texas House who was following the hearing from afar, took an opposite lesson. “Tell me again how arming our teachers is your solution to the gun violence problem,” he wrote on Twitter. “The problem is the GUNS.”

The outline presented by Mr. McCraw confirmed details first reported by The New York Times over a series of articles during the past month, including that the officers who first made it inside the school — two minutes after the gunman — had AR-15-style rifles, and that shields that could have been used to protect officers making an entry into the classroom had arrived before 12 p.m., nearly an hour before officers finally went in.

Mr. McCraw also presented new details, such as the exact time that Chief Arredondo went into the school, at 11:36 a.m., three minutes after the gunman entered the classrooms and began firing.

The timeline also noted that, by 11:54 a.m., a Texas Ranger was inside the school, one of at least 12 members of the State Police who responded between the time when the gunman began shooting in classrooms at 11:33 a.m. and when officers killed him at 12:50 p.m.

The presentation contrasted starkly with the version of events offered by Chief Arredondo in an interview with The Texas Tribune. The Times has reported that Mr. Arredondo had arrived at the school without his police radio and focused on finding keys to the classrooms, even though it was not apparent in the videos that anyone had checked the classroom door to see if it was locked.

Chief Arredondo said that the classrooms had been locked and that he knew this because he and another officer had checked both doors. He said he then focused on finding keys, testing dozens of them, he said, in an effort to find one that would work on the doors. Eventually one was located, he said, and was used by the team that entered the classroom and killed the gunman.

But Mr. McCraw said there was no indication, either from video or interviews, that anyone had in fact checked the doors. “Moreover, you don’t need a key,” he said, pointing to the availability of breaching tools and the possibility of entering through the windows.

Tuesday’s hearing represented the first public comments on the investigation in several weeks.

The Department of Public Safety stopped holding public briefings within a week of the shooting after several of the details shared by officials, including Mr. McCraw and Gov. Greg Abbott, turned out to be incorrect. The information that had be corrected included the length of time it took for officers to fire the first shots at the gunman (not immediately, but one hour and 17 minutes after he began shooting inside the school) and how he had gained access to the building (not through a door that had been propped open, but through one that was unlocked.)

Instead of providing updates, the State Police began referring media inquiries to the local district attorney, Christina Mitchell Busbee, who has declined requests for interviews and has not held any news conferences.

The shifting narrative surrounding the massacre, which left 19 children and two teachers dead, quickly undermined trust in the official accounts of the shooting and created tension between state officials and those in Uvalde, most of whom rallied around their city Police Department and Chief Arredondo, who recently took a seat on the City Council.

Those tensions only grew when Mr. McCraw held a news conference three days after the shooting and said that Chief Arredondo had been in charge of the police response and had made the “wrong decision” in not trying to immediately confront the gunman.

Soon after that news conference, on May 27, the mayor of Uvalde, Don McLaughlin, requested that the federal Justice Department conduct its own investigation, independent of the one by the Texas Rangers. The State House is also conducting its inquiry, meaning there are now at least three investigations into what happened.

Without official briefings, details emerged by other means, including through investigatory documents, surveillance video and transcripts of police body camera recordings reviewed by The Times.

The Times revealed that police supervisors had been told there were people alive but wounded in the classrooms; that an officer had been on the phone with his wife, a teacher, after she was shot but before she died, and that he had told other officers about this at 11:48 a.m., providing them with a clear indication that people inside the classrooms were in urgent need of help; and that a Uvalde police officer passed up an opportunity to take a shot at the gunman outside the school, fearing he might hit children.

On Tuesday, several senators asked whether the delay had cost lives. Most of the gunfire took place in the first few minutes that the gunman was inside the classrooms, though he fired several additional shots as officers waited outside the classrooms.

“Is there any way to determine how different this outcome might have been had we been able to go in right away?” Senator Bryan Hughes, a Republican from Tyler, asked toward the end of the hearing.

Mr. McCraw said the district attorney was very interested in answering that question and had sought assistance from experts in emergency medicine to do so.

The final question for Mr. McCraw came from Senator José Menéndez, a Democrat from San Antonio, and focused not on the police who responded, but on the AR-15-style rifle wielded by the gunman.

“Based on the 100 rounds that he shot in a short time period, could he have done as much damage with a bat or a knife or a revolver?” Mr. Menéndez asked. “Could he have killed as many people?”

“No,” Mr. McCraw answered.