The Las Vegas police again revised the timeline of the Oct. 1 shooting at the Mandalay Bay Resort and Casino, saying on Friday that Stephen Paddock shot and wounded a security guard on his hotel floor about the same time he began to spray bullets on a country music concert below.

The gunman shot the guard, Jesus Campos, around 10:05 p.m., Sheriff Joseph Lombardo of the Las Vegas Metropolitan Police Department said at a brief news conference on Friday, not at 9:59 p.m., as he had told reporters on Monday, and not after Mr. Paddock stopped firing on the crowd at 10:15, as the sheriff said on Oct. 4.

The latest account came a day after MGM Resorts, which owns Mandalay Bay, disputed the police department’s timeline, saying in a statement that the time of 9:59 had come from a hotel report “manually created after the fact” and was not accurate.

The sheriff did not say on Friday whether Mr. Paddock had already begun shooting at the crowd from the window of his 32nd-floor suite when he wounded Mr. Campos, or turned to the concert immediately after. MGM said in its statement that the gunman was firing at the concert grounds “at the same time as, or within 40 seconds after” the time Mr. Campos first reported shots over his radio.