The two boys were shot around 7:30 p.m. in the 3600-block of 167th Street, near Phrommer Park in Hammond, Indiana, police said.
When police arrived, a 13-year-old boy, later identified by the Lake County Coroner's Office as Thomas Delacruz from East Chicago, was pronounced dead at the scene, officials said.
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The other boy was shot in the leg and was taken to a nearby hospital for treatment before later being released.
Police said the shooting happened after trick-or-treating in Hammond had officially ended for the night.
No other details regarding the shooting have been released.
The nature of the incident is under investigation but a person of interest is in custody for questioning, police said. No charges have been filed.
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In a statement, Hammond police said the arrest was possible because of help from the community. "Our hearts go out to the families of these boys," police said in the statement.
Anyone with information is asked to call the Hammond Police Department.