Authorities also partially drained the 80-acre lake to better see the shoreline.
On Thursday night, Ian Ritch, father of Maddox Ritch, made an emotional post on Facebook about his son.
"There is no sense of accomplishment here today".
Officials said the body was found in an area thick with underbrush.
Search crews had checked the same area as recently as Tuesday. The medical examiner would formally identify the body. FBI Special Agent Jason Kaplan said.
"We know a lot of people were in the park yesterday and we have spoken to many of them, but we have not spoken to everyone".
"Everybody looks at you as a monster, and I regretted that since the moment its happened", he said.
"We still want to hear from the others", Kaplan said. Maddox's parents have been notified by law enforcement.
The boy's father said his son, Maddox Ritch, disappeared Saturday as they were walking in the park.
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"If you were at Rankin Lake yesterday and saw Maddox or took video or photos of your outing at Rankin Lake Park, we would ask that you call us", he said during a news conference Sunday, according to WBTV.
"He initially took off just a little ways, maybe 25-30 feet from me", Ian Ritch said.
His father, Ian Ritch, reported that the boy ran ahead of him and vanished in broad daylight. Rich told ABC that Maddox frequently does so, but always stops and waits for him to catch up. "I feel guilt for letting him get so far ahead of me before I started running after him", Ritch said.
Police and the Federal Bureau of Investigation are reporting that a body found around 1:00 p.m.in a creek is likely that of a missing six-year-old. After an hour, Ritch called 911, saying he delayed that call because he thought he would find his son and there was no reason to call police. Officers say they want to make sure searchers employ every available tool to search the lake.
He was last seen wearing an orange t-shirt with the words "I'm the man" written on it. The father told WJW-TV that he has trouble running because he has diabetes and suffers from neuropathy in his feet.
"This is not the ending we had hoped for", said Gastonia Police Department Chief Robert Helton on Thursday. Earlier in the investigation, the authorities said they had evidence that Maddox was in the park, but did not specify what evidence they had.
Kaplan said investigators were still looking into Maddox's previous whereabouts, his movements, and how he ended up there.
Children with autism are often prone to wandering or bolting away.
Maddox was described as nonverbal, which complicates the search for him.
Speaking on Wednesday, Mr Ritch said: 'He likes running.