Authorities also found items belonging to Laundrie, like a backpack and notebook, officials said.
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FBI Special Agent in Charge Michael McPherson said the area where the items were found had previously been underwater. McPherson said a team would be on site for several days processing the scene.
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The medical examiner's office in nearby Sarasota was called the preserve earlier, and local police asked a cadaver dog team to assist in the search.
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"Chris and Roberta Laundrie went to the Myakkahatchee Creek Environmental Park this morning to search for Brian. The FBI and NPPD were informed last night of Brian's parents' intentions and they met Chris and Roberta there this morning. After a brief search off a trail that Brian frequented some articles belonging to Brian were found. As of now law enforcement is conducting a more thorough investigation of that area," Stephen Bertolino, the Laundrie family attorney, said in a statement.
Petito, 22, went missing on a cross-country trip with Laundrie, and authorities confirmed that she was strangled to death after her body was discovered in Wyoming last month. Laundrie is a person of interest in her killing.
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She was reported missing on Sept. 11 by her parents after she did not respond to calls and texts for several days while she and Laundrie visited national parks in the West.
The case generated enormous public interest but also raised uncomfortable questions over the unequal attention given to the hundreds of cases of Native American and other minority women missing or murdered across the United States. Petito is white.
Petito's body was found Sept. 19 on the edge of Wyoming's Grand Teton National Park, which the couple had visited. The coroner there concluded she died of strangulation and her body had been where it was found for three or four weeks.
The couple was stopped on Aug. 12 by police in Moab, Utah, after they had a physical altercation, but no charges were filed.
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Laundrie returned home alone on Sept. 1 in the Ford van the couple took on their trip. He was reported missing after telling his parents on Sept. 14 that he was going for a hike in the Carlton Reserve, a nature preserve in Sarasota County that has for weeks been a key area in the search.
He is charged in a federal Wyoming indictment with unauthorized use of a debit card, which would allow authorities to arrest him if he is found alive.
It alleges Laundrie used a Capital One Bank card and someone's personal identification number to make unauthorized withdrawals or charges worth more than $1,000. It does not say to whom the card belonged or what type of charges were made.
ABC News and the Associated Press contributed to this report.