(CNN) The six-day search for Maddox Ritch, the 6-year-old boy with autism who went missing at a North Carolina park, ended Thursday afternoon with the discovery of a body in a creek, officials said.

"I'm heartbroken," Gastonia Police Chief Robert Helton told reporters at an emotional news conference.

"Our community is heartbroken, our searchers, our investigators. This is not the end we had hoped for."

Since Saturday, when Maddox took off running while on a walk with his father around the lake at a Gastonia park, authorities had employed dogs, drones, sonar and infrared in the search for the boy. They partially drained the 80-acre lake so they could better see its shoreline. Gastonia SWAT teams searched the lake and nearby waterways on kayaks.

"Maddox Ritch, a beautiful young man, blond hair, blue eyes, out walking this past Saturday with his dad, waking around the lake, looking at turtles, doing the things that a young man would do and this is not the end we hoped for," said Helton, wiping a tear at one point.

"Still, a lot of unanswered questions."

The body was found by a team of 15 searchers in a creek about 1 mile east of Rankin Lake Park, where Maddox was last seen with his father and a friend, Gastonia Fire Chief Phil Welch said. About 180 people had been out searching on Thursday.

"There is no sense of accomplishment here today," Welch said. "There is grieving down at the command post."

The body was partially submerged in 2 to 3 feet of water. The area, thick with underbrush, had been searched numerous times.

Identification of the remains will be done by the medical examiner's office, but authorities indicated they believe they are of Maddox. His parents were notified soon after the discovery.

Jason Kaplan, an FBI supervisory special agent, said investigators want to determine the cause and manner of death as well as the boy's exact movements.

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. — FBI Charlotte (@FBICharlotte) September 27, 2018

"We still want to hear from the people who were in the park that day," he said.

Kaplan declined to comment on the condition of the body.

"I think it's reasonable to say that it's him but some time will pass before we are able to say that definitively," Kaplan said.

Asked if there were signs of foul play, Kaplan said it's too early to tell.

Earlier Thursday, the FBI in Charlotte tweeted, "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."

Maddox Scott Ritch

On Saturday, Maddox took off running during a walk around the lake at a Gastonia park, a police spokeswoman said. Maddox's father, Ian Ritch, said he and a friend were walking 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 would run 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.

911 call: 'We searched everywhere'

Ian Ritch said he, the friend who was with them and park staff members first helped search for Maddox. Chief 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.

Blue-eyed 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 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.

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

Authorities hoping to talk to jogger

Helton, the Gastonia police chief, said earlier this week that 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.

How did 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 was described as 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.

Mourning Maddox

On Thursday night, mourners gathered for a vigil in Gastonia in Maddox's memory. Some wore shirts bearing his name, a cross and a heart. A chaplain prayed. People cried and sang a hymn.

Some in the group had gathered before for two consecutive nights, praying for Maddox's safe return.

"The prayer tonight's more for his family and for his soul to leave peacefully now to go up to our Lord and Savior to be in his arms now ... in the ultimate Kingdom, to run around as play freely as he wishes," Mikaela Moose told CNN affiliate WSOC.

In a message on Facebook, Ian Ritch wrote: "Now today i found out im not a dad anymore. I had big plans with my son."

"I will now and forever be a broken man until i take my last breath," he wrote.