A 3-year-old boy was found dead in a septic tank after playing in a backyard over the weekend, police say.

The Pierce County Sheriff’s Department told FOX 13 Seattle that the young child – who was a Lakewood, Washington, resident – was pronounced dead after being removed from the tank at around 3 p.m. on Monday. Authorities were alerted to the incident at around 2:20 p.m. that day.

Investigators believe that the boy was playing with his 4-year-old brother and six-year-old neighbor before he fell into a neighbor's septic tank. The child reportedly removed the tank's lid before falling in.

His brother quickly went inside to alert their grandmother, who then called the police.

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The Pierce County Sheriff’s Department says that a child died after falling into a septic tank. (iStock / Google Maps)

"Detectives and forensic investigators were called to the scene to document the incident," Sgt. Darren Moss said in a statement.

Authorities told FOX 13 that an adult was "in and out of the house with them."

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File photo of a Pierce County Sheriff’s Department vehicle. (Pierce County Sheriff’s Department via Facebook)

The Tacoma News Tribune reported that the tank was "similar to the size of a sewer manhole," measuring roughly three feet in diameter.

Fox News Digital reached out to the Pierce County Sheriff's Department for more information, but did not immediately hear back.

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File image of a septic tank. (iStock)

Authorities are actively investigating the incident. No additional details are known at this time.