The Hilliard police motorcycle officer who was killed in a crash Thursday afternoon lost control of his bike on a freeway ramp, struck a barrier and fell to his death from the overpass, witnesses said.

The Hilliard police motorcycle officer who was killed in a crash Thursday afternoon lost control of his bike on a freeway ramp, struck a barrier and fell to his death from the overpass, witnesses said.

Investigators said Friday that they don't know what caused Officer Sean Johnson, described by his peers as a seasoned rider, to veer into a concrete wall on the curving, flyover ramp from Interstate 270's southbound lanes to the eastbound lanes of Route 161.

"A police (officer) on a motorcycle just hit the rail and went off onto 161," said one 911 caller. "He hit the guardrail and flipped over."

Johnson, who was participating in a training ride on the freeway with three other motorcycle officers, is the first Hilliard officer to die in the line of duty in the division's 56-year history.

"We've got a hole in our organization that will never be filled," Chief Robert Fisher said Friday afternoon as he stood beside a division cruiser covered with flowers in the city's First Responders Park, a memorial that he referenced as "someplace I never thought we would gather to honor one of our own killed in the line of duty."

>>> Video: Escort for fallen Hilliard officer

The 46-year-old father of two died after crashing his Victory Stealth motorcycle at 1:44 p.m. Thursday in the cloverleaf interchange in Blendon Township. Fisher said he believed the crash was a "freak accident."

Franklin County Chief Deputy Jim Gilbert, who oversees the sheriff's office patrol division, said the investigation will take time.

"We don't know why he drifted," Gilbert said. The four officers were riding as a group, and "we don't think speed is a factor," he said. "There is no indication that anyone was operating in an unsafe manner."

No other vehicles were involved, and there were no reports of debris in the roadway or unusual road conditions that might have played a part. Johnson was wearing a helmet. After the crash, the other Hilliard officers rushed to Johnson in a grassy area about 30 feet below the ramp and tried to revive him.

"It looks like (he) fell off the overpass," said another 911 caller. "There were two people doing active CPR on him. I know they were doing CPR because I'm a nurse."

The officers were training on motorcycles that the division received earlier this year, but Hilliard Officer Hyda Slone said Johnson was not new to riding.

"Sean was one of the most experienced riders in the group," she said.

Late Friday afternoon, dozens of police vehicles escorted Johnson's body from the Franklin County coroner's office to a funeral home Downtown. His calling hours are planned for 4 to 8 p.m. Tuesday at St. Paul the Apostle Church in Westerville, with a funeral service to follow there at 11 a.m. Wednesday.

Johnson, a U.S. Air Force veteran, worked for the attorney general's Ohio Investigative Unit and as a Fairfield County deputy before joining Hilliard in 1999. The Investigative Unit remembered him as "a soft-spoken professional and a good man."

His children are 11 and 14.

"I have teenage kids myself," Fisher said. "And to think what them losing their dad is doing to them, it's horrible."

tdecker@dispatch.com

@Theodore_Decker