An Iranian helicopter carrying five people crashed into the Caspian Sea early Sunday, killing all aboard, according to reports. The chopper belonged to the National Iranian Oil Company and was transporting a person suffering a cardiac arrest from an offshore oil platform.

The helicopter crashed off the coast of Behshahr city in the east of Mazandaran province in central-northern Iran. The crash reportedly occurred due to a technical malfunction.

Authorities have not recovered the chopper’s black box yet, according to Iran’s Press TV, which added this was second helicopter crash in Mazandaran this year.

In September, an air ambulance chopper on a rescue mission crashed on a mountainous route that links Tehran to the Caspian Sea shores. A medical emergency crew member was killed in the accident.

Air accidents are common in the Islamic republic, where aircraft are reportedly poorly maintained.