Robert Card, 40, a member of the US Army Reserves, is alleged to have opened fire at a restaurant and a bowling alley Wednesday evening in which at least 18 people were killed and 13 others injured, Xinhua news agency reported.
The suspect was found dead at 7:45 p.m. local time (2345 GMT) Friday evening near the Androscoggin River in Lisbon Falls, about 10 miles (16 km) from Lewiston, an official of the Maine Department of Public Safety said, adding that he had an apparent self-inflicted gunshot wound.
An arrest warrant has been issued for Card for eight counts of murder, Maine State Police said Thursday.
Local authorities rescinded the shelter-in-place order for neighboring communities in the wake of the suspect's death.
US President Joe Biden has spoken to state officials to offer his support after the deadly shootings, and ordered all United States flags lowered to half-staff until Monday to honor the victims, the White House said on Thursday.
The massacre was the 565th shooting in the US in 2023 and the deadliest so far this year, according to the Gun Violence Archive, a nonprofit research group.
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Maine, a northeastern state of about 1.3 million population, has one of the lowest homicide rates in the US, according to the US Centers for Disease Control and Prevention.