Police did not immediately identify the man, and Lanier did not discuss the circumstances of the discovery.
"We know it's an adult male. It appears to be a suicide," Lanier said.
Eight-year-old Relisha Rudd was last seen on March 1 in the company of Kahlil Tatum, a janitor at the homeless shelter where she lived with her mother and brothers. Relisha's mother had allowed her to spend time with Tatum, and her family did not report her missing. Repeated absences from school led authorities to start looking for her on March 19.
The following day, the body of Tatum's wife was found in a motel in nearby Oxon Hill, Md., dead of a gunshot wound to the head. He has been charged in a warrant with murder.
On March 2, police said Tatum purchased 42-gallon trash bags, and he was seen that day in the area of Kenilworth Aquatic Gardens, a 700-acre park.
Police started searching for Relisha in the park on Thursday in an operation aimed at recovering a body, although Lanier said she had not given up hope that Relisha was alive. Hundreds of people, including divers, have participated in the search.
"Our primary focus here was to find Relisha," Lanier said Monday. "We're not finished. That search is continuing."
Police issued an Amber Alert after the discovery of Tatum's wife's body, and digital billboards along the East Coast displayed photos of Relisha and Tatum.
