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Police monitoring Brian Laundrie last month thought he had come home. It was his mom.

Investigators said they were surprised when Laundrie's parents reported him missing. They thought they had seen him return home.

Police in North Port, Florida, have admitted that they made a mistake while monitoring Brian Laundrie's movements from his home last month.

Investigators trained cameras on the Laundrie residence after his fiancée, Gabby Petito, was reported missing by her family in New York on Sept. 11. Laundrie had returned to Florida without her from their cross-country road trip Sept. 1, police said.

Those monitoring the home, where Laundrie lived with his parents, saw him leave in his Mustang on Sept. 13 and thought they saw him return Sept. 15, according to a report from WINK News in Fort Myers, which police confirmed to Telemundo in Tampa. Telemundo and NBC News are both owned by NBCUniversal.

“All I’m going to say is we know where Brian Laundrie is at,” North Port Police Chief Todd Garrison said Sept. 16.

On Monday, police acknowledged they were mistaken. The person who came back in the Mustang wasn't Laundrie, North Port Police spokesperson Josh Taylor told the news station. It was Laundrie's mother, Roberta, who was wearing a baseball cap, according to the official.

“They’re kind of built similarly," he said.

Officials realized the mix-up when the family reported Sept. 17 that Laundrie was missing, according to police.

"That was certainly news to us that they had not seen him,” Taylor said. “We thought that we seen Brian initially come back into that home on that Wednesday. But we now know that that wasn’t true.”

He added: "The area was difficult to keep an eye on with the growing activity. He was still not wanted for a crime at the time."

As authorities looked for Laundrie, 23, focusing largely on the vast Carlton Reserve, a nationwide search was also ongoing for Petito, 22.

Her body was found Sept. 19 at the Spread Creek Dispersed Camping Area in the Bridger-Teton National Forest in Wyoming. Petito had been dead for at least three weeks, and her death was ruled a homicide by "manual strangulation," the coroner said.

Laundrie was never charged in Petito's death, but he was named a person of interest in her disappearance. Authorities also issued an arrest warrant for him after they said he had used Petito's debit card without permission.

Human remains found in Florida's Carlton Reserve last Wednesday were confirmed to be Laundrie's after a review of dental records, officials said Thursday. His parents helped lead the FBI and North Port police to the Myakkahatchee Creek Environmental Park, a part of Carlton Reserve.

According to the FBI, the remains were found alongside personal items belonging to Laundrie, such as a backpack and a notebook. The contents of those items have not been disclosed.

The remains have been sent to an anthropologist "for further evaluation" after autopsy results came back inconclusive, the family's attorney, Steven Bertolino, said late last week.

"No manner or cause of death was determined," he said in a statement to NBC News.

Bertolino on Monday said he was not given a timeline for when the anthropologist will conclude the evaluation of Laundrie's remains, which he said will ultimately be cremated.

He added that Laundrie's family has no immediate plans for a funeral, although it may decide on a private ceremony later.

"The family is grieving privately somewhere in Florida," he said.

Petito and Laundrie left for their trip from Blue Point, New York, in early July in a 2012 Ford Transit van. The couple chronicled their trip on their Instagram accounts and YouTube under the moniker Nomadic Statik. Petito stopped communicating with her family in late August.

The case continues to generate enormous public interest, but it has also raised questions about the unequal media and law enforcement attention given to missing white women compared to missing people of color.