A William Floyd High School senior and his passenger were killed Thursday when the suspected stolen dirt bike they were riding in North Bellport was struck after being pursued by a minivan driven by the brother of the bike’s owner, Suffolk County police said.

The driver of the dirt bike, Keenen King, 19, of Shirley was pronounced dead at the scene, police said. The other victim, Anthony Holmes-Garriques, 20, of North Bellport was initially taken to Brookhaven Memorial Hospital Medical Center in East Patchogue and was transferred to Stony Brook University Hospital — a major trauma center — where he died Thursday.

Christopher Bouchard, 27, of Mastic Beach, was arrested after the 10:15 a.m. crash on Montauk Highway near Doane Avenue, said Suffolk County Police Commissioner Timothy Sini. His passenger and brother, Brian Bouchard, 31, was not detained. Brian Bouchard is the owner of the dirt bike, police said.

Christopher Bouchard was charged with first-degree reckless endangerment. He was being held Thursday at the Fifth Precinct and is scheduled to be arraigned at First District Court in Central Islip on Friday.

Although witnesses told Newsday the Honda Odyssey minivan was speeding and appeared to intentionally strike the two young men on the dirt bike, Sini said that aspect of the incident is under investigation.

“With heavy hearts, the William Floyd school community mourns the loss of high school senior Keenen King, who died in a motor vehicle accident this morning,” James Montalto, a school district spokesman, said in a statement Thursday.

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“Even though school is out, grief counselors will be available for students to speak with. The superintendent has ordered flags to half-staff throughout the district. Keenen’s family and friends are in our thoughts and prayers at this time.”

Montalto said King was scheduled to graduate in August.

A distraught man who identified himself as King’s father went to the scene of the crash Thursday, and cried out: “That’s my son. I don’t care what he did, he did not have to die.” He declined to speak to reporters at the scene.

The series of events leading to the tragedy began early Thursday. Bouchard called police about 7 a.m. to report his dirt bike stolen from his Mastic Beach home, police said.

Christopher Bouchard saw two young men on his brother’s bike around 10 a.m. Thursday and pursued them, ultimately striking the bike, Sini said.

“We believe that dirt bike was the one reported stolen earlier in the day,” Sini said. “So far, the evidence indicates the individual claiming to be the owner of that dirt bike came upon two men on that dirt bike and pursued the dirt bike, culminating in the crash . . . Some evidence indicates the driver of the minivan after the crash tried to put the bike in the back of the van.”

Witnesses described a violent scene in which the two young men were knocked off the bike and the driver of the minivan attempting to load the dirt bike into his vehicle while the two young men lay critically injured in the roadway.

Charles Parker, 69, of Middle Island, said he was driving west on Montauk Highway behind a dirt bike when he saw a speeding eastbound minivan cross the double-line and hit the bike carrying two people.

Parker said the minivan pulled away, did a U-turn and stopped near the scene and the van driver and his passenger tried to put the dirt bike in the van, but they were prevented by other witnesses.

Parker said he blocked the van with his car to prevent the suspects from fleeing. He said he called 911 and spoke to police after the crash.

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The accident forced the temporary closure of Montauk Highway between Station Road and Hoffman Avenue while police investigated.

Still, mourners came to the scene throughout the day.

Freddie Parker, who said he’s a cousin of Holmes-Garriques, said he happened upon the aftermath of the crash and he and others prevented the minivan’s occupants from leaving. He said he tended to his injured cousin as he lay on the concrete.

“ ‘Yo Anthony, you OK?’ ” Freddie Parker, 43, of North Bellport, recalled telling him. “ ‘Just blink your eyes or something. Let me know you’re OK.’ . . . The other kid, he took his last breath right there. Laying on the ground.”

King’s girlfriend, Trinity Clendenning, 16, of Mastic, broke down in the middle of the street crying.

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Her cousin Stephanie Landon, 20, said King loved to drive on two wheels.

“He was one of those kids who would ride anything,” she said.

With John Valenti and Mark Morales