By Jason Hanna and Greg Botelho
ATLANTA (CNN) -- Truck driver Gerald Smith glanced to his right and saw a small plane flying low and toward him as he motored along a busy Atlanta-area highway Friday morning.
Smith hit the brakes, an action he thinks saved his life. The plane's occupants couldn't save their own.
The single-engine Piper PA-32 passed in front of Smith's decelerating tractor-trailer and crashed on Interstate 285 just north of Atlanta, killing all four people aboard and severely snarling traffic on the bypass that encircles the city, officials said.
No one on the ground was injured, DeKalb County Fire Department spokesman Capt. Eric Jackson said, even though the plane hit the highway shortly after rush hour, at about 10 a.m.
"Whoever was in that plane, my heart goes out to their family," Smith told CNN Atlanta affiliate WSB. "Just a normal workday for me (is now) a day I'll never forget, because somebody's life was taken."
Grady Gideon Byrd told CNN that his son, 53-year-old Greg Byrd, and two grandsons, Christopher, 27, and Phillip, 25, were on the plane.
Christopher's fiancee, Jackie Kulzer, also was on board, according to the elder Byrd.
The four on board were headed to the University of Mississippi for graduation ceremonies Saturday for Greg Byrd's younger son, Robert, according to the Jackson Clarion-Ledger.
According to the Asheville Citizen-Times, Greg Byrd, the pilot, was a small-business owner there. Before that, he served in the Buncombe County Sheriff's Office.
"He loved life, he was adventurous. He really did a great job at the sheriff's office," Sheriff Van Duncan told the newspaper.
The crash happened shortly after the craft left DeKalb Peachtree Airport, the Federal Aviation Administration said.
All lanes in both directions were temporarily closed at the crash site, causing long traffic backups for hours.
The only sign of damage that remains is a large black burn mark on the highway's median.
The plane appeared to have crashed into a concrete barrier separating five eastbound and five westbound lanes, DeKalb County Police Chief Cedric Alexander said. The wreckage smoldered near the barrier.
The cause of the crash wasn't immediately clear, National Transportation Safety Board investigator Eric Alleyne said. The NTSB will reconstruct the plane in an effort to determine what happened, he said.
Witnesses said they saw the plane flying low before hitting the highway, creating a fireball that sent a thick plume of smoke into the air.
Smith, the truck driver, told WSB that he was driving when he looked to his right and "saw a plane approaching me at a fast rate of speed (and) flying very low."
He slammed on his brakes. The plane clipped the front of his truck but continued past him, he said.
"It wasn't any higher than the top of my truck," Smith told WSB. "By the time I hit the brakes, I heard the impact on the front of my truck."
"If I would have stayed on the gas, I probably would have hit it head on," Smith said.
Driving on nearby Peachtree Industrial Boulevard, Don McGee saw the plane nearly clip a red light before it reached the highway. He turned onto I-285 to see "large plume of black smoke (and) fire everywhere."
"I ran across the field to see what was going on and see if I could help somebody, but it was gone by then, it was done," McGee told WSB. "It was like a bomb being dropped or something."
Siva Chamarthy also got out of his vehicle, but the intense heat stopped him in his tracks.
"I was thinking of the people inside (the plane), I couldn't hear anything," Chamarthy said, according to WSB. "It was so hot, I couldn't go near them."
The NTSB's Alleyne said he didn't yet know whether the pilot told an air traffic controller anything that would have foreshadowed the crash.
Alexander, the county police chief, said it was "quite remarkable, considering the fact (this happened) on 285 around that time in the morning, that it was not any more tragic than it already is."
CNN's Martin Savidge, Shawn Nottingham, Joshua Berlinger and Marylynn Ryan contributed to this report.
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