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, authorities said.

CARLINVILLE, Ill. — 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, authorities said.

Investigators will examine the wreckage to determine if there was any mechanical failure as well as study the plane's maintenance history and the pilot's history, the National Transportation Safety Board said in a preliminary report released this week.

Found at the scene of the wreckage was a GoPro camera, which federal investigators are studying for clues. It may be more than a year before the cause of the crash can be determined, authorities said.

The single-engine plane crashed May 31 near Carlinville, about 50 miles (80 kilometers) north of St. Louis, Missouri. The pilot, Joshua Daniel Sweers, 35, of Michigan, had taken off from Creve Couer Airport outside St. Louis that afternoon.

Sweers and three others on the plane were all engineering graduates from Kettering University in Flint, Michigan, and belonged to the Phi Gamma Delta fraternity, according to the St. Louis Post-Dispatch. The passengers were identified as Daniel A. Shedd, 37, of St. Charles, Missouri; Daniel Schlosser, 39, of Michigan; and John S. Camilleri, 39, of New York.