In this July 7, 2018 photo, a helicopter that crashed near Interstate 57 and Interstate 94 sits in a field of grass between the two interstates in Chicago. A pilot may have saved lives by crashing a medical helicopter in a grassy area on the South Side of Chicago rather than on major interstate highways or a train station nearby, officials said Sunday. (Armando L. Sanchez/Chicago Tribune via AP)

In this July 7, 2018 photo, a helicopter that crashed near Interstate 57 and Interstate 94 sits in a field of grass between the two interstates in Chicago. A pilot may have saved lives by crashing a medical helicopter in a grassy area on the South Side of Chicago rather than on major interstate highways or a train station nearby, officials said Sunday. (Armando L. Sanchez/Chicago Tribune via AP)

CHICAGO (AP) — A pilot may have saved lives by crashing a medical helicopter in a grassy area on the South Side of Chicago rather than on major interstate highways or a train station nearby, officials said Sunday.

The four people onboard Saturday evening were injured but no one was hurt on the ground.

The Federal Aviation Administration said the crash occurred around 9:15 p.m. Saturday near the intersection of three expressways.

The patient in the helicopter was taken to a hospital in critical condition, said Chicago Fire Deputy District Chief Walter Schroeder, while the three crew members onboard were transported in stable condition.

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The crash caused a snarl in Chicago traffic, with northbound I-57 and southbound I-94 lanes closed Sunday while the investigation continues.

Schroeder said the pilot issued a “mayday” call before the crash, but the exact cause is not yet known. He commended the pilot for putting the aircraft down in a grassy median away from traffic.

Deputy district fire chief Lynda Turner said the pilot maneuvered the helicopter to land belly-down with all rotors intact. The helicopter was smoking but not on fire, she said.

“The pilot did an excellent job of landing a helicopter that was in an emergency situation,” Turner said at a press conference.

The helicopter was headed toward Gary Airport. It’s unclear where the flight originated.

The FAA and The National Transportation Safety Board are investigating the cause of the crash. Results of the FAA report will not be made public until the investigation concludes, which could take up to a month, spokesman Tony Molinaro said. Peter Knudson, a NTSB spokesman, said its investigation will be finished in one to two weeks and that the wreckage site will be cleared later on Sunday.