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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"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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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.

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