(CNN) — Four people were killed, including a child, when a helicopter slammed into a radio tower behind homes in Houston’s Second Ward, causing a fiery explosion and toppling the structure and scattering debris through the neighborhood, local officials said.

The crash happened near Engelke Street and North Ennis Street just before 8 p.m. Sunday, igniting a large explosion that could be heard at a fire station nearly half a mile from the site, Houston Mayor John Whitmire said during a late-night news conference.

There were four people onboard the privately-owned Robinson R44 helicopter, according to Houston Police Chief Noe Diaz and federal officials. Whitmire confirmed all four were killed, CNN affiliate KTRK reported.

“The preliminary information we have is that the helicopter was operating as an air tour flight when it crashed into a radio tower under unknown circumstances,” the National Transportation Safety Board said in a statement Monday.

The aircraft appears to have departed the Ellington Airport, which is about 17 miles south of the crash site, but its destination was unclear, the mayor said.

While the cause of the crash remains unclear, NTSB officials are expected at the crash site Monday to examine and recover the aircraft, the agency told CNN.

The Federal Aviation Administration said it is also investigating.

Surveillance video from a nearby home obtained by CNN shows what appears to be the helicopter flying toward the tower, then exploding on collision, lighting up the sky.

Witness video shows fire personnel urging onlookers to clear the area where a fire was burning and warning them of a nearby gas tank.

“This is a tragic event tonight. It’s a tragic loss of life,” Diaz said.

The fire was no longer active as of 9:30 p.m., a fire department spokesperson said.

“The residents are secure and safe, but we have a terrible accident scene,” Whitmire said.

Photos shared by the fire department show the mangled tower draped across a dirt clearing behind a line of homes.

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

Some homes near the crash site lost power, Whitmire said.

The mayor said the city is “fortunate” that the explosion was not more severe because there is a gas tank in the area.

Manuel Arciniega was nearby when the crash occurred, according to CNN affiliate KPRC.

“My buddy behind me told me to look up in the sky, and I just saw the cell tower crumbling down. They said a helicopter hit it,” Arciniega told the affiliate.

“It will be a large investigation because of the expanse of the accident,” Diaz said. If residents find helicopter parts in the area, they should avoid touching them and notify fire or police officials, he added.

CNN’s Lauren Mascarenhas, Rosa Flores and Sara Weisfeldt contributed to this report.

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