The Union County Homicide Task Force is collecting evidence because the deaths were unattended.
Union Township police pulled 3-year-old Kendell Francois and his 5-year-old brother, Kenley, from the bottom of the pool. They were pronounced dead at a hospital at 8:17 p.m. Thursday.
The Union County Prosecutor's Office says it appears the boys climbed a 4-foot fence to get in the pool.
It's not clear how long they were in the pool or were away from adult supervision.