In an emotional news conference after the discovery, Gastonia Police Chief Robert Helton tearfully told reporters, "this is not the end that we had hoped for."
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Rankin Lake Park will remain closed until police can answer questions about what exactly happened after Maddox, who is autistic and nonverbal, went missing.
FBI supervisor Special Agent Jason Kaplan said at the news conference that the investigation is not over.
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"You should not take away anything from that except to understand that, in law enforcement, we will not take anything for granted," he said. "So there is a lot of work and a lot of people who have lost a lot of sleep committed to continuing to making sure we answer every question that we can about where Maddox was, how his movement occurred and how it was that he ended up where he did."
He added that it was amazing that Maddox was found.
"It was extremely difficult to see him even when we were standing right next to him," Special Agent Kaplan said.
Authorities said the depth of water was about 2-3 feet where the body was found, but there are various depths in the creek. He was partially submerged in the water.
FBI officials said it is too early to say whether foul play was a factor.
Police still want to talk to witnesses who were in the park on Saturday, the day Maddox disappeared. Specifically, authorities said they want to talk to a man who was loading a silver kayak into a white truck. The man was reportedly wearing a camouflage hat.
Maddox went missing on Saturday, Sept. 22, at Rankin Lake Park in Gastonia. The city released a 911 call made by a park employee.
According to the boy's father, Ian Ritch, Maddox was about 25 feet to 30 feet away before he broke into a sprint just as a jogger passed them. The father said he is a diabetic and, because he has neuropathy in his feet, he has trouble running.
"He likes running," Ritch said. "I couldn't catch up with him. I feel guilty for letting him get so far ahead of me before I started running after him."
Formal identification of the body will be done by the medical examiner's office.
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