Four people were killed and one critically injured Sunday in a hot air balloon crash in Arizona, police said.

The tragic incident happened at approximately 7:50 a.m. in a desert area in Eloy, a small town with a population over 15,000, and some 65 miles northwest of Phoenix known as “the skydiving capital of the world.”

The cause of the crash was not immediately known.

The National Transportation Safety Board and Federal Aviation Administration were leading the investigation.

The identities of the victims were being withheld pending notification of their next of kin.

It was not immediately clear if or how the victims knew each other.

The Eloy Police Department confirmed by email that all of those on board the balloon were adults but declined to offer additional specifics pending the result of the investigation.

“The Eloy Police Department regretfully confirms that there are four deceased individuals and one person in critical condition as a result of this incident,” the department said in a statement.

“Further information will be provided as it becomes available. We thank the community for their support and understanding as we navigate this tragic event.”

Eloy is home to two parachute manufacturers: Firebird USA and Sun Path Products, as well as the world’s largest skydive drop zone, Skydive Arizona, which is located around five miles from the site of the balloon crash.

An employee answering the phone declined to comment late Sunday afternoon.

This story is developing.