Authorities continue to investigate the circumstances surrounding a helicopter crash in Houston on Sunday evening that killed four people on board and sparked a massive flame within sight of homes.

The privately owned Robinson R44 helicopter collided with a radio tower after taking off from Ellington Airport, about 17 miles south of the crash site.

Investigators are now looking into whether malfunctioning lighting on the tower played a role in the crash, which toppled the structure.

The ages and names of the pilot and three passengers killed have not been released, but officials confirmed one of the passengers was a child.

On Monday, Brian Rutt, an air safety investigator with the National Transportation Safety Board, described the scene of the crash as “complex” and said the investigation is in its early stages.

Here’s what we know:

Initial moments of the crash

The helicopter was flying at an altitude of approximately 600 feet when it struck the 1,000-foot radio tower just before 8 p.m. Sunday, according to investigators and open-source data.

No one on the ground was injured and nearby homes were undamaged, although some vehicles were affected, officials said. The crash ignited a fire that scorched approximately 100 to 200 yards of grass.

The crash near Engelke Street and North Ennis Street, just east of downtown Houston, resulted in a significant explosion, Mayor John Whitmire said at a Sunday night news conference.

Firefighters from a nearby station who heard the crash quickly responded, Whitmire said. He described the scene as a “terrible accident” and noted that both the tower and the helicopter were destroyed. Some nearby residents experienced power outages, he added.

While watching a football game on Sunday night, Lawrence Gurule heard a noise he said “sounded like a bomb.”

“I ran outside and looked up and I could see smoke. The tower was gone,” he said.

Lighting may have failed

The lighting on the radio tower is believed to have been malfunctioning for several days prior to the crash. The Federal Aviation Administration issued a notice Thursday indicating the tower’s lights were deemed “unserviceable” until the end of the month.

FAA guidelines state towers over 200 feet above ground level “should typically be marked and/or lighted,” and any outages in tower lighting “should be addressed as quickly as possible,” as outlined in a 2020 publication.

Towers and their guy wires – which are used to stabilize a structure – present a particular danger to helicopters, which usually operate at low altitudes, and can be difficult for pilots to spot at night.

Video footage of the crash shows at least one flashing light at the top of the tower, but the remainder of the structure appears unlit.

Investigators searching for debris

While the cause of the crash is unknown, the NTSB told CNN additional investigators were set to arrive at the site Monday evening to examine and recover the aircraft.

The flight was identified as a “sightseeing” tour, with the helicopter registered to Porter Equipment Holdings LLC, based in Magnolia, Texas, according to the FAA. CNN has reached out to the company for a response.

Law enforcement was searching for debris in the area Monday, and the Houston Police Department’s vehicular crimes division was performing a three-dimensional scan over a 4-acre area to provide data to both the NTSB and FAA, Lt. Jonathan French said during a Monday news conference.

Officials advised residents who see debris around the crash site to leave it alone and report it to authorities.

CNN’s Pete Muntean, Elizabeth Wolfe, Sarah Dewberry, Lauren Mascarenhas, Rosa Flores, Sara Weisfeldt and Jillian Sykes contributed to this report.