Try 1 month for 99¢

In 1933, Melvin Purvis — who in less than a year would become famous for the capture of bank robber and escape artist John Dillinger —stepped in to investigate the downing of United Flight 23. On a transcontinental flight, the Boeing 247 was on its way from Newark, N.J., to Oakland, Calif., and scheduled to make a stop in Chicago. But around 9 p.m., just five miles outside of Chesterton witnesses saw an explosion tear off the plane’s tail, sending it plummeting 1,000 feet to the ground where it exploded once again. All seven people aboard died.

At first county officials theorized a broken gas line or ruptured motor on the transcontinental flight triggered the blast. But Porter County coroner Dr. Carl Davis had another theory — a time bomb caused the explosion. Though other authorities disputed Davis’s assertion, he was backed by coroner Dr. C.W. Muehlberger, a member of Northwestern University's Crime Detection Laboratory who determined the plane had been carrying a bomb that most likely contained nitroglycerine.

"Our investigation convinced me that the tragedy resulted from an explosion somewhere in the region of the baggage compartment in the rear of the plane,” said Purvis, then head of the Chicago office of the FBI. “Everything in the front of the compartment was blown forward, everything behind blown backward and things at the side outward. The gasoline tanks, instead of being blown out, were crushed in, showing there was no explosion in them.”

The crash and what caused it stirred the interest of Bryan W. Alaspa, a Chicago writer and author of "Sabotage: A Chronicle of the Chesterton Crash" (to be released for Kindle, Nook and e-readers in June).

"It’s pretty amazing how little is known about it considering that it is a fairly historic event,” Alaspa said. “Not many people know about it in the area or in Chicago."

Alaspa first discovered information about the plane crash when researching a book he was writing about Silas Jayne, a Chicago businessman thought to be involved in the disappearance of three young women from the Indiana Dunes back in the 1960s.

“When I came across it, I became really intrigued,” Alaspa said. “I’m the kind of person who loves an unsolved mystery and trying to find out answers. It’s been sort of a morbid fascination for me for a long time.”

A diligent researcher, Alaspa interviewed relatives of those who saw the crash and perused documentation provided by the Westchester Township Historical Museum. He also filed his first Freedom of Information Request with the FBI, ending up with more than 300 pages of reports that weren’t available through any other source.

"I had reports signed by Melvin Purvis and J. Edgar Hoover that outlined suspects and investigative tangents that agents had to follow to try and find the culprit,” said Alaspa, author of other books such as "Ghosts of St. Louis: The Lemp Mansion and Other Eerie Tales," "Chicago Crime Stories: Rich Gone Wrong" and the soon-to-be released "Chicago Unsolved Mysteries." “It was fascinating. The reports even included some of the crackpots theories like a guy who said he was psychic and knew it wasn’t a bomb but a leak in the cabin and another who said it was a meteor that hit the plane.”

Alaspa’s research also was a lesson in early 20th century aviation. The Boeing 247 had just been introduced that year and is considered to be the first modern passenger airliner. United was the first airline to hire female flight attendants and a requirement was that they also be nurses. Alice Scribner, 26, of Chicago, was a trained nurse just starting her new job. She holds the unlucky designation of being the first United flight attendant to be killed in a plane crash. Others killed in the explosion included a young woman whose fiancé had finally persuaded her to fly out to Nevada so they could get married.

“The theory that seemed to be most accepted around that time was that the mob was trying to assassinate a U.S. attorney who was notorious for prosecuting bootleggers and was known to make the cross-country flight on a regular basis."

Another prevalent thought was that the bomb was planted by a union sympathizer intent on smearing the reputation of the airline because of their fight against unionization but no evidence ever emerged to support that theory.

The mystery remains unsolved.