Authorities say a Syracuse house fire that killed four children and two adults was an accident caused by smoking materials left on a porch.

Fire Chief Paul Linnertz said Thursday that investigators might never know what those materials were. He says witnesses who reported an explosion were most likely hearing windows breaking as flames spread quickly through the two-story home May 6.

Linnertz says there were no working smoking alarms.

The children were two sets of siblings who were ages 7 to 13. The dead also included the father of two of those children and an adult cousin.

Calling hours for the victims have been set for 11 a.m. to 3 p.m. on May 18 at the Thomas J. Pirro Funeral Home in North Syracuse. Their funerals will be private.