AURORA, Ill. (AP) — An investigation into a medical helicopter crash that killed a desperately ill 1-year-old girl and three crew members will include whether a radio tower’s lights were on when the aircraft clipped the structure’s wire and went down in a suburban Chicago field, authorities said.

Investigators also have pieces of the helicopter, including a mangled yellow-and-white rotor, to determine what happened to the Air Angels helicopter that crashed minutes before midnight Wednesday.

The crash caused so much damage to the 734-foot tower that Aurora police advised about 1,000 people who live nearby to leave their homes until engineers believe it is safe.

The helicopter was flying about 50 feet lower than the top of the tower when it hit the wire, said National Transportation Safety Board investigator John Brannen.