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PHILADELPHIA (CNN)[Breaking news update, posted at 2:07 p.m. ET]

A body believed to be that of an autistic 6-year-old boy who went missing after his father lost sight of him in a North Carolina park has been found, the FBI in Charlotte tweeted Thursday. “With heavy hearts we announce a body believed to be 6-year-old Maddox Ritch was found at approximately 1 p.m. off of Marietta Street/Old Dallas Highway in Gastonia. Maddox’s parents have been notified of the discovery. The investigation is ongoing.”

[Previous story, posted at 12:09 p.m. ET]

Six-year-old Maddox Ritch went missing last week after his father lost sight of him when he took off running in a North Carolina park. The search for the missing boy, who has autism and is nonverbal, entered its sixth day on Thursday.

Here is what we know:

How Maddox disappeared

Maddox took off running during a walk around the lake, a police spokeswoman said.

Maddox’s father, Ian Ritch, said he and a friend were walking with Maddox in the park when his son ran ahead, likely triggered by a passing jogger. Maddox was about 25 to 30 feet away when he broke into a sprint, Ritch said.

Ritch said he “was giving him just a little leeway, freedom” when he let him run up ahead, but he had a clear view of his son.

Maddox often runs ahead of him but would usually slow down and stop to give him time to catch up, Ritch said.

“I couldn’t catch up with him. I feel guilt for letting him get so far ahead of me before I started running after him,” Ritch told reporters on Wednesday.

The search for Maddox

Ritch said he, the friend who was with them and park staff members first helped search for Maddox. Gastonia Police Chief Robert Helton said a part-time park employee reported Maddox missing.

According to a 911 call released by police, a caller told authorities Maddox’s parents had been searching for him for almost an hour.

“We searched everywhere,” the man said on the phone call.

Maddox is 4 feet tall and weighs 45 pounds. He has blond hair and blue eyes. Maddox was last seen wearing an orange T-shirt with the words “I am the man,” along with black shorts and closed-toe sandals.

Federal, state and local authorities have chased more than 150 leads, conducted hundreds of interviews, gathered surveillance videos from local stores and employed dogs, drones, sonar, ATVs and infrared technology in the search for Maddox.

Authorities also partially drained the 80-acre lake to better see the shoreline. The FBI, meanwhile, has brought in its underwater search and evidence response team.

Gastonia Fire Chief Phil Welch said authorities have brought in new people every day to search a 250-acre park area.

Ritch spent some of Tuesday at the park with authorities retracing their steps, the FBI said.

What Maddox’s parents say

Maddox’s parents have encouraged anyone who may have information on the case to call the special tip line, 704-869-1075.

His mother, Carrie Ritch, said her son has a contagious smile and laugh. He loves the park, bouncy balls and his teddy bear, she said.

“Continue praying for him because I just want my baby home, please, whatever you can do,” she said. “Maddox is my whole world and my reason for living. He’s mama’s boy.”

Maddox’s father said not being able to find his son has “been torture.”

“I’m not eating. I’m not sleeping. I’m just worried about getting my little boy back,” he said.

What authorities say

Helton, the Gastonia police chief, said authorities want to talk to an unidentified male jogger who was in the park at the time and a professional photographer whom witnesses saw taking photos of other children dressed in Dr. Seuss costumes.

Hundreds of people were in the park on Saturday, including other blond-haired boys also wearing orange T-shirts, Helton said.

“But we need to know if you saw our blond-hair with the orange shirt,” Helton said.

The police chief said any piece of information from potential witnesses will help authorities put together a timeline.

Authorities are also hoping a $10,000 reward offered by the FBI will provide information to help find Maddox.

“We don’t want another hour to pass before we find him and bring him home,” Helton said this week.

How does Maddox’s autism affect the search?

Children with autism are often prone to wandering or bolting away. When children with autism go missing, rescuers get to know their likes and dislikes, and use familiar sounds, such as the voice of a cartoon character and a favorite song to draw them out during searches.

Maddox is nonverbal, which complicates the search for him. Hoping he would respond to his parents’ voices, authorities pumped prerecorded messages into Rankin Lake Park.

“Many of these children are more likely to respond to a favorite character, a unique interest or familiar voice,” said Lori McIlwain, a co-founder and board member of the National Autism Association.

(The-CNN-Wire™ & © 2018 Cable News Network, Inc., a Time Warner Company. All rights reserved.)