(DETROIT FREE PRESS) - A St. Clair County teenager celebrating her high school graduation Saturday night died, along with her brother, when an enormous bonfire exploded in their backyard, investigators said.

Savannah Blewett, 18, was attending the party with family and friends who were piling a large amount of wood and brush about 100 feet from the house at 6500 block of Gratiot in rural St. Clair Township. Someone poured a large amount of gasoline on the wood and her brother, Christopher Blewett, 27, invited her to light it.

The resulting explosion killed them both and injured four others, three of whom were hospitalized. Their conditions were not available Sunday afternoon.

The explosion sent wood flying 100 yards and blew out the windows of the house, investigators said.

"Everybody is still trying to understand what happened," Sgt. Matt Stringer of the St. Clair County Sheriffs Department said today.

On Facebook, someone created a page called "R.I.P. Savannah and Christopher Blewett," and posted, "You lost your lives in an accident that should have never happened. We all love and miss you dearly."