The sound – a tremendous clap of noise like thunder – let him know something was wrong.
And when Matthew Scull turned to look over Tampa Bay, his eyes confirmed what his ears had heard: Something terrible had happened.
A helicopter's rotor had apparently malfunctioned. A horrified Scull watched as a small black helicopter plummeted toward the bay, its white rotor racing it to the waters below.
"The helicopter just dropped and it did a flip,'' said Scull, 41. "It hit the water and a big splash.''
The helicopter landed a few hundred yards from the shore in Apollo Beach and quickly sank. The Hillsborough County Sheriff's Office said the helicopter only had one person in it, though Scull swears he saw two.
The crash happened during good weather, Scull said, while the helicopter was maybe 75 yards above the bay.
"I've never seen anything like that,'' Scull said. "I just feel bad. I saw them. I'm definitely going to be thinking about that for a few days. I just watched two people die.''
Sheriff's Office spokesman Larry McKinnon said the crash was first reported shortly after 3 p.m. by an off-duty Tampa police officer. The officer said he witnessed the crash and saw a body floating in the water.
Several private boats immediately raced to the site of the crash and were quickly joined by numerous boats from the sheriff's office, Marine Patrol and U.S. Coast Guard. The helicopter was not visible from above but was located by underwater sensors.
Federal Aviation Administration officials said the craft was a small, two-person Robinson "R22" helicopter.
McKinnon said searchers' hope of finding a survivor dimmed as the minutes turned to hours.
"It looks like it's turning into a (body) recovery effort now,'' McKinnon said at about 7 p.m.
Scull said he also had little hope anyone survived. When he saw the crash, he jumped into his boat with his daughter and nephew and sped to the scene. "We saw little bits of debris,'' he said.
By mid-evening, the sheriff's office had learned who owned the helicopter and who had been the pilot who had taken out the helicopter. The sheriff's office did not immediately release the pilot's name, pending notification of relatives and the recovery of the body.
The sheriff's office said the helicopter was privately owned. Scull said he saw the helicopter flying in the area most days, cruising up and down the coastline.
Nightfall delayed the search for the pilot and the effort to recover the helicopter. The search was called off about 9:30 p.m. The search will resume at 7:30 a.m. Saturday.
Phil Richardson was near a marina at Apollo Beach and was among those watching the flurry of activity in the area as helicopters and boats searched the area Friday afternoon.
"There were at least six or eight boats hunting around trying to locate him," Richardson said. "Guys at the marina said they heard a 'pop' and the engine speed up and that was it."