MADISON, Wis. -- Prosecutors anticipate a man accused of killing three teenagers at a northern Wisconsin swimming hole will plead insanity.

Marinette Count District Attorney Brent DeBord has yet to file formal charges against Scott J. Johnson, 38, of Kingsford, Mich.

But in a letter sent to the state Justice Department asking for help prosecuting the case, DeBord said he expects Johnson to enter a not guilty by reason of mental defect plea.

The letter doesn't elaborate on why DeBord anticipates that plea, and the district attorney didn't return messages Tuesday afternoon.

Police said they believe Johnson stepped out of the woods on Thursday and gunned down Tiffany Pohlson, 17, Anthony Spigarelli, 18, and Bryan Mort, 19. All three teens were from Michigan.

Johnson surrendered Friday morning after an all-night manhunt, but he has yet to be formally charged. Charges against him are expected sometime this week.

Justice Department spokesman Bill Cosh said the assistant attorney general will not take the lead on the case, instead assisting DeBord.

Funeral services for Pohlson and Spigarelli were held in Michigan Tuesday. Mort's funeral will be held on Thursday.

Marinette County Coroner George Smith said Pohlson and Spigarelli were killed from single gunshot wounds to the head. But he didn't know what type of gun was used or how far the shooter was from the teens.

Autopsies for Pohlson and Spigarelli were conducted in Wisconsin because their bodies were found on the Wisconsin side of the Menominee River.