An enormous, important discrepancy has emerged over what happened during the Las Vegas massacre: When did police arrive on the 32nd floor where Stephen Paddock was firing his deadly fusillade onto concertgoers below?

Las Vegas police say they didn’t get to the floor until after the shooting was over. But MGM Resorts International, the owner of Mandalay Bay, says police officers were there shortly after the shooting began, responding to a report of a security guard being shot. The discrepancy could raise questions about whether police might have taken steps to intervene while Paddock was launching his devastating 10-minute onslaught.

Sheriff Joe Lombardo, the head of the Las Vegas Metropolitan Police Department, amended the timeline of events slightly on Friday, saying that Mandalay Bay security guard Jesus Campos first arrived on the 32nd floor at 9:59 p.m., but wasn’t shot until about 10:05 p.m. That is also, Lombardo said, when Paddock began his attack on the concert. “We still stand by that time,” Lombardo said in a news conference, where he took no questions.

The next point in the sheriff’s timeline: 10:17 p.m. “Twelve minutes” later, Lombardo said. “That is when our officers first arrived on the 32nd floor.” By then, Paddock had stopped firing. He was found dead an hour later, having killed 58 people and wounded hundreds more.

But on Thursday, in response to inquiries about when Mandalay Bay notified police of the Campos shooting, MGM Resorts issued a statement that was unequivocal: Las Vegas police officers accompanied Mandalay Bay security to the Campos shooting and “immediately responded.” MGM said that “Metro officers were together with armed Mandalay Bay security officers in the building when Campos first reported that shots were fired over the radio. These Metro officers and armed Mandalay Bay security officers immediately responded to the 32nd floor.” The statement says MGM believes Paddock began firing out the window of his room within 40 seconds of Campos reporting his shooting, and Lombardo said Friday, “I agree with the statement.”

In addition, Mandalay Bay engineer Stephen Schuck also arrived on the 32nd floor as the shooting began, and told the “Today” show that Campos told him to take cover as rifle shots whistled down the hallway. A recording of Schuck’s radio transmissions shows that he called in his report of someone firing a rifle on the 32nd floor, and in less than a minute he reports, “Security is here.” There is no time stamp accompanying the recording.

Schuck told the “Today” show that police were also there, corroborating the MGM statement, as was his supervisor. He said he took the supervisor’s master keys, rode the elevator to the basement and turned off the elevators, then returned to the 32nd floor to give the police the keys “so they could use them.”

Schuck did not specify if these police arrived with Mandalay security or were the officers who arrived at 10:17 p.m. after searching the hotel for the shooter. Lombardo has previously said that police did not learn of the Campos shooting until their officers, scouring the hotel floor-by-floor for the shooter, arrived on the 32nd floor.

A transcript of Las Vegas police dispatch broadcasts, captured by Broadcastify, does not show any mention of a guard being shot on the 32nd floor until 10:18 p.m., when a dispatcher says, “Mandalay security says shooter is on 32nd floor, they have a security officer on that floor who was shot.”

But were Las Vegas officers already there? If they had arrived at the beginning of the shooting, they would probably not have been equipped to take on a gunman firing automatic-style weapons, who had already sprayed the hallway with what Lombardo has said were 200 rounds. Police theories on active shooters now advocate going in immediately to try to save lives, but patrol officers with semiautomatic pistols would be no match for the rifles Paddock was firing. Could other steps have been taken? Flash grenades, use of tear gas or pepper spray gas, or other disruptive measures could have been options, but also wouldn’t have been carried by standard patrol officers making the first response to the scene.

When did Mandalay Bay notify the police of the Campos shooting? In response to a question about that Thursday, MGM released its statement, implying that since police were there from the outset there was no need for further notification. Officers accompanied Mandalay Bay officers to the Campos shooting, MGM said. Were the officers then aware of the carnage Paddock was creating nearby? Radio transcripts indicate plenty of traffic once the shooting into the concert began. But Lombardo has maintained that his officers didn’t know where the shots were coming from until they climbed the stairs to the 32nd floor.

Either MGM or the Las Vegas police are incorrect about when officers first arrived on the 32nd floor. The discrepancy is crucial to knowing what happened in the hotel while the shooting was occurring, and whether it could have been stopped.

Here is the full text of MGM Resorts’ statement Thursday:

Although we prefer not to comment on the details of the investigation, we are issuing this statement to correct some of the misinformation that has been reported. The 9:59 p.m. PDT time was derived from a Mandalay Bay report manually created after the fact without the benefit of information we now have. We are now confident that the time stated in this report is not accurate. We know that shots were being fired at the festival lot at the same time as, or within 40 seconds after, the time Jesus Campos first reported that shots were fired over the radio. Metro officers were together with armed Mandalay Bay security officers in the building when Campos first reported that shots were fired over the radio. These Metro officers and armed Mandalay Bay security officers immediately responded to the 32nd floor. We will continue to work with law enforcement as we have from the first moments of this tragedy as they work toward developing an accurate timeline.