The lights on a Houston radio tower were reportedly “unserviceable” a few days before a helicopter crashed into it, killing four people in a fiery explosion.

According to investigators, the flight was operating as an air tour, flying at an altitude of 600 feet when it slammed into the 1,000-foot-high tower over the weekend.

The Federal Aviation Administration issued a notice to pilots Thursday stating the tower’s lights were “unserviceable” until the end of the month.

However, surveillance video of the crash shows at least one light flashing at the top of the tower, with the rest of the tower in total darkness before the helicopter exploded on collision.

Guidance from the FAA says towers higher than 200 feet “should normally be marked and/or lighted” and an outage “should be corrected as soon as possible.”

The privately owned Robinson R44 II helicopter crashed Sunday night, causing a large explosion near Engelke Street and North Ennis Street.

According to Houston Police Chief J. Noe Diaz, the helicopter either struck a cable or the tower, and no one on the ground was injured.

Fire Chief Thomas Muñoz added no residents were impacted, but the fire was estimated to have spanned two to three blocks before it was extinguished.

The identities of the victims and their ages have not been released.

The National Transportation Safety Board is in charge of the investigation and said officials will be at the crash site to begin the process of documenting the scene and examining the aircraft.

Lt. Jonathan French said Monday that the Houston Police Department’s vehicular crimes division is making a three-dimensional scan of a 4-acre area to share with the NTSB and FAA.

A preliminary report is expected to be released in 30 days and will contain factual information gathered during the initial phase of the investigation.

“A probable cause of the crash, along with any contributing factors, will be detailed in the final report, which is expected in 12 to 24 months,” the NTSB stated.