The authorities in Florida announced on Saturday that they were scouring a vast wildlife reserve for a man who had been declared a “person of interest” in the disappearance of his fiancée, one day after his parents disclosed that they hadn’t seen him since Tuesday.
The North Port, Fla., police said on Twitter they were searching the “vast Carlton Reserve,” a 24,565-acre park, for the man, 23-year-old Brian Laundrie.
The F.B.I. and officers from other agencies were helping in the search, the police said, as a search for Mr. Laundrie’s fiancée, Gabrielle Petito, continued in Wyoming — the woman’s last known whereabouts, according to her family.
The search for Ms. Petito involved “ground surveys” in Grand Teton National Park, the F.B.I. said on Saturday.
Mr. Laundrie returned to his parents’ home in North Port alone on Sept. 1 from a cross-country van trip that he had taken with Ms. Petito, 22. Her parents reported her missing 10 days later.
The police said they learned on Friday night that Mr. Laundrie was missing after detectives went to the home in North Port that Mr. Laundrie shares with Ms. Petito and his parents.
Mr. Laundrie’s parents had summoned the police to the home, according to Josh Taylor, a spokesman for the North Port police.
“They are now claiming they have not seen their son since Tuesday,” Mr. Taylor said on CNN on Friday night. “It is another twist in the story, for sure.”
Mr. Laundrie’s family have said they believe that he entered the Carlton Reserve this week, the police said on Twitter.
The reserve, which is about 13 miles north of North Port, is a public park owned by Sarasota County that is home to a diverse array of wildlife including feral hogs, alligators and panthers. On Saturday, the reserve’s website warned that most of the park’s 80 miles of hiking trails were flooded. There has also been “some unusual fire caused by lightning,” said Russell Johnson, a longtime volunteer with the park.
“All in all, this is the worst time of the year for visiting the Carlton,” he said.
Mr. Taylor told reporters that more than 50 police officers were searching for Mr. Laundrie on foot, on all-terrain vehicles and with the help of dogs. He described the conditions as wet and muddy and said the park had been closed to the public while the police searched. Mr. Taylor said Mr. Laundrie’s life could be in danger, but he did not specify how.
“I think that’s fair to say,” he said. “There is an enormous amount of pressure on him to provide answers on what’s going on here.”
Mr. Taylor said that when Mr. Laundrie’s family contacted the police on Saturday they did not “share any information about Gabby” or what Mr. Laundrie may have told them about her whereabouts.
Their focus was giving the police information they would need “to help us locate their son,” Mr. Taylor told Fox 13 News.
In a statement on Friday night, Mr. Taylor said the police, who have been under public pressure for days to question Mr. Laundrie, were frustrated that he was gone.
“For six days, the North Port Police Department and the F.B.I. have been pleading with the family to contact investigators regarding Brian’s fiancée, Gabby Petito,” he wrote. “Friday is the first time they have spoken with investigators in detail.”
Mr. Taylor said that Mr. Laundrie remained a person of interest in Ms. Petito’s disappearance.
“He is not wanted for a crime,” Mr. Taylor wrote. “We are not currently working a crime investigation.”
Rather, he said, the department is pursuing “multiple missing person investigations.”
Steve Bertolino, a lawyer for Mr. Laundrie, said in a statement to several news outlets that the F.B.I. had removed property from the home in an effort to find Mr. Laundrie. The F.B.I. did not immediately respond to a request for comment.
Through Mr. Bertolino, Mr. Laundrie had declined to speak with investigators, the police said. He has not been arrested or declared a suspect in the case; the police have described him only as a person of interest, a vague term often used by law enforcement agencies to identify a person who they believe may have been involved in a crime.
In an interview broadcast on Friday on the ABC program “Good Morning America,” Mr. Laundrie’s sister Cassie Laundrie said she had not spoken to him since he returned to his home in North Port, which is near Fort Myers.
“I wish I could talk to him,” she said.
Before Mr. Laundrie went missing, Mr. Taylor had said the police had no reason to arrest him after he returned to Florida in the van, which is registered to Ms. Petito and had not been reported stolen.
“The reality of that situation is that it was a common-use vehicle between the two of them,” Mr. Taylor said. The state law does not allow the police to arrest Mr. Laundrie any more than it allows for the arrest of a teenager found driving his or her parents’ car, he said.
F.B.I. agents, National Park rangers and police officers in at least two states have been searching for Ms. Petito since her parents reported her missing on Sept. 11.
Her father, Joseph Petito, said in an interview released on Friday by the North Port police that family members had gone to Wyoming because it was “the last known place that we have.”
“We’ve got to start somewhere,” he said. “There’s 2,500 miles between Wyoming and Florida, so if we start now it’s less we have to do when we pinpoint a better location. That’s how we look at it. What if we get lucky, and we don’t need Brian anymore, and we find her right now?”
Ms. Petito left with Mr. Laundrie in July in a white Ford van outfitted for a cross-country adventure, according to the police.
In the video that the police released on Friday, Mr. Petito called on people who know Brian Laundrie’s parents to put pressure on them to say something.
“I’ll let the courts and, you know, society judge them,” he said. “I already did my judgment, it’s not going to change. So now I’m focusing on what matters. I just hope people pay attention. I hope people look.”
The case has drawn widespread attention, as reporters have gathered outside Mr. Laundrie’s house and members of the public have scoured the couple’s Instagram accounts, which depicted a seemingly carefree, nomadic “Van Life” in the American West.
Recently, people began gathering outside of the home of Mr. Laundrie’s parents to protest. Some stood on the lawn, holding signs that demanded to know where Ms. Petito was. Others drove by in golf carts holding signs and one man stood in front of the house with a megaphone and yelled at the family to “let them know where Gabby is.”
Ms. Petito and Mr. Laundrie left New York on July 2 for what was supposed to be a four-month cross-country trip visiting national parks, said Ms. Petito’s stepfather, Jim Schmidt.
Mr. Laundrie posted on Instagram that they were “downsizing our life into this itty bitty van.” Ms. Petito posted that converting a Ford Transit into a camper was “an adventure in itself.” The van was decorated with plants and outfitted with a bed, tiny bookcases and a small wooden counter to prepare food.
On Aug. 12 in Moab, Utah, Mr. Laundrie had “some sort of argument” with Ms. Petito and told her to take a walk and calm down, according to the Moab police, who responded to a report of a “domestic problem.”
Mr. Laundrie and Ms. Petito both told the police that they were in love and engaged to be married and “desperately didn’t wish to see anyone charged with a crime.”
Mr. Laundrie told an officer that “issues between the two had been building over the last few days,” a police report said.
Ms. Petito cried during the encounter with the police and said she suffered from anxiety, according to body camera footage of the episode. In the police report, Ms. Petito is recorded saying she moved to slap Mr. Laundrie because she feared that he “was going to leave her in Moab without a ride.”
Both told the police that the episode should be classified as a “mental/emotional health ‘break,’” rather than as a domestic assault, according to the report.
She admitted hitting Mr. Laundrie, who was calm when he spoke with the officers, politely declining offers of bottled water because he disliked plastic.
In the report, the police described Mr. Laundrie as the victim of the incident. They arranged for him to stay in a hotel that night while Ms. Petito kept the van. No charges were filed, the report states.
On Instagram, Ms. Petito kept a personal blog of their journeys. In many of the posts published before her disappearance, she was photographed smiling and posing against backdrops of nature.
Ms. Petito, the oldest of six siblings, had been working as a pharmacy technician to save money for the trip. She met Mr. Laundrie at Bayport-Blue Point High School on Long Island, Mr. Schmidt said. They began dating after graduation and moved two years ago to Florida, he said.
Johnny Diaz contributed reporting.