A truck driver fatally struck a bike messenger Monday morning in Manhattan without stopping — but eventually returned to the scene, insisting he never saw her.

The rider — identified by officials as Robyn Hightman — was pedaling between cars along Sixth Avenue near West 24th Street around 9:35 a.m. when the Freightliner slammed into her and kept going, authorities and witnesses said.

“The whole right side of her head was crushed,” said a construction worker who witnessed the horrifying aftermath, as well as a responding firefighter’s vain efforts to save the cyclist.

“The fireman didn’t want to give up, and I didn’t want him to give up either.”

Paramedics rushed the 20-year-old to Bellevue Hospital, where she was pronounced dead on arrival.

Meanwhile, the driver of the Freightliner returned to the scene, claiming he only kept going because he had no idea he’d struck Hightman until a cabbie flagged him down.

“I didn’t see her. A taxi waved me down a block away and said I hit someone,” said the distraught trucker, who only gave his name as Antonio. “I looked and I didn’t see anyone.”

A woman who answered the phone at the hauling company listed on the truck’s cab, New Jersey-based A&E Stores, Inc., denied that their driver ever left the scene before declining further comment.

As NYPD investigators scoured the rig for physical evidence, the driver passed a Breathalyzer test.

He was briefly driven away by police, before returning to his truck with five summonses, authorities said.

Authorities are withholding the cyclist’s name, pending family notification of her death.

Roughly 20 bike messengers raced to the scene as word of the fatal smash-up spread, and demanded justice in the wake of this and other recent cyclist deaths, threatening to “shut down” the city in protest.

“There’s no accountability for drivers,” said one courier, Mario Sepulveda. “How many deaths do we have to have for something to be done?”

Police did not immediately release Hightman’s name, but she was mourned in a Facebook post by fellow members of a pro-cycling team for which she served as an “ambassador.”

“Robyn’s application to our program was the most passionate, in-depth one we’ve received out of hundreds,” read the remembrance posted by the Hagens Berman Supermint pro-cycling team — which included a portion of that piece.

“As a homeless youth deeply entrenched in the trappings of poverty and parental abuse and neglect, my first bicycle offered a way to seek respite from the horrors of my surroundings and human experience, if only for a few short minutes,” wrote Hightman.

“While riding, I’ve been hit by cars, physically and sexually assaulted, cycled through hurricanes and sub zero temperatures and small tornadoes (seriously), been injured, and become lost; the list goes on,” Hightman also once said, according to the post.

“Although to some extent I am still plagued by feelings of intimidation as well as the fear of being seen as less legitimate than other cyclists due to my lack of proper training, I will ride on. I am no longer scared.”