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.

A helicopter that crashed into a Houston radio tower, killing all four people aboard and starting a fire below, was on an air tour flight at the time, authorities said.

NTSB says they are in the early phases of their investigation into the deadly helicopter crash and collecting evidence from the scene in Houston’s Second Ward.