A group is ending a 20-year search for a plane that crashed into Lake Michigan in 1950, killing all 58 people on board, after sweeping the vast body of water using sonar technology and even getting support from an acclaimed adventure writer.

When Northwest Orient Flight 2501 crashed, it was the worst aviation disaster in U.S. history.

Valerie van Heest, executive director of the Michigan Shipwreck Association, said she has mixed feelings about ending the search, which began in 2004.

“It’s a hard thing to have to say because part of me feels like we have failed,” van Heest told The Detroit News, “but we have done so much to keep memory of this accident and these victims at forefront that I feel like we’ve done better for them than if we’d found the wreckage.”

After covering 700 square miles of Lake Michigan, Van Heest said scientists believe the plane broke up into pieces too small to be detected by side-scan sonar and likely “sunk into the muck” on the bottom.

While searching for the wreckage from 2005 to 2013, the nonprofit found nine shipwrecks, but not Flight 2501.

“MSRA independently continued the effort for the next 10 years to complete coverage of a 600 square mile search grid, locating two unmarked graves where victims’ remains were buried, but not the wreckage,” the group said in a social media post.

The plane, a propeller-driven DC-4, left LaGuardia Airport in New York at night on June 23, 1950, with two stops planned on the route to Seattle. An intense storm suddenly appeared and the plane went down.

According to the Michigan Shipwreck Association, the pilot was 35-year-old Captain Robert C. Lind and the co-pilot was Verne F. Wolfe, also 35, and the flight attendant was 25-year-old Bonnie Ann Feldman. The 55 passengers included 27 women, 22 men and six children, the nonprofit said.

Debris and body parts washed ashore in South Haven, Michigan.

“We know this plane hit the water with great force, and we know there was no way to survive this,” said van Heest, who has written a book about the mystery, “Fatal Crossing.”

Clive Cussler, an author whose adventure fiction has sold in the millions, financially supported a search until 2017. Also known for his own shipwreck hunting and underwater exploits, Cussler died in 2020.

“I hope someday the families of those lost will have closure,” Cussler wrote in 2018.