Northwest Orient Airlines Flight 2501 was a DC-4 propliner operating its daily transcontinental service between New York City and Seattle when it disappeared on the night of June 23, 1950. The flight was carrying 55 passengers and three crew members; the loss of all 58 on board made it the deadliest commercial airliner accident in America at the time. The aircraft was at approximately over Lake Michigan, NNW of Benton Harbor, Michigan when flight controllers lost radio contact with it soon after the pilot had requested a descent to . Witnesses reported hearing engine sputtering noises and a flash of light after the last radio transmission. A widespread search was commenced including using sonar and dragging the bottom of Lake Michigan with trawlers, but to no avail. Considerable light debris, upholstery, and human body fragments were found floating on the surface, but divers were unable to locate the plane's wreckage. Cause It is known that Flight 2501 was entering a squall line and turbulence, but since the plane's wreckage underwater was not found, the cause of the crash was never determined. There is output from a hindcast simulation of the possible weather conditions during the event. The incident was reported as follows: Aftermath The missing airliner is the subject of an annual search by Michigan Shipwreck Research Associates (MSRA), a Michigan-based non-profit organization. The search effort began in 2004 as a joint venture between author and explorer Clive Cussler and the MSRA. Cussler ended his involvement in 2013, but sent his side scan sonar operator back to Michigan in 2015, 2016, and 2017 to follow some leads discovered by MSRA. In September 2008, MSRA affiliate Chriss Lyon, investigating the crash of Flight 2501, found an unmarked grave that contains the remains of some of the 58 victims. Valerie van Heest, MSRA co-director and author of the book Fatal Crossing, says human remains from the June 1950 crash into Lake Michigan washed ashore and were buried in a mass grave. She claims they were buried in a St. Joseph-area cemetery without the knowledge of the victims' families, and the grave was never marked. In a 2008 ceremony at the cemetery with 58 family members of Flight 2501, a large black granite marker, donated by Filbrandt Family Funeral Home, was placed in Riverview Cemetery that now lists the names of the 58 and the words "In Memory of Northwest Flight 2501, June