CARLINVILLE, Ill. (AP) — Authorities say a small plane that crashed last month while heading from Missouri to Michigan, killing four men aboard, entered into a “right descending spiral” before it went down in a southwestern Illinois farm field.

The National Transportation Safety Board says in a preliminary report that investigators will examine the wreckage to determine if there was any mechanical failure.

They’ll also study the plane’s maintenance history and the pilot’s history. The single-engine plane crashed May 31 near Carlinville.

The St. Louis Post-Dispatch reports the four men who died were engineering graduates from Kettering University in Flint, Michigan, and belonged to the Phi Gamma Delta fraternity.