The attackers opened fire on a Jewish deli and killed four, including a cop.

Jersey City Police Detective Joe Seals never suspected the white, nondescript U-Haul van parked in Bayview Cemetery posed a threat.

Seals did not draw his weapon as he approached the van. He never had the chance to report the arsenal and explosives inside.

Without warning, David Nathaniel Anderson, 47, and Francine Graham, 50, immediately opened fire, killing Seals, a father of five.

“It’s a loss to the department and it reinforces the fragile nature of the job police do on a day in, day out basis,” Jersey City Mayor Steven Fulop told ABC News in an interview to mark the one-year anniversary of the domestic terrorist attack that left six dead and was targeted at a Jewish deli. “At any moment things can turn for the worst very quickly.”

Investigators theorize the chance encounter with Seals — who had just finished meeting an informant — may have accelerated the killers’ plans.

When they shot Seals, the killers had not yet fully assembled a pipe bomb that was later found in the van and that the FBI concluded had the capacity to kill or injure people up to 500 yards away.

Instead, infused with hate — and armed with an AR-15 semiautomatic assault rifle, a Mossberg .12-gauge shotgun and Glock .9mm pistols — the killers drove to a nearby Jewish day school where 40 boys were deep in study.

They ended up blasting away at the adjacent kosher market, killing the owner, Mindy Ferencz, employee Douglas Rodriguez and a customer, Moshe Deutsch, before they were killed in an hour-long gun battle with police.

“Prior to the shooting on Dec. 10th, we did not see any of the precursors that you generally see, anti-Semitic acts or hate crimes,” Fulop said. “We didn’t see graffiti or tagging or swastikas.”

There was an undercurrent of tension between the Black and Orthodox Jewish communities, but Anderson and Graham lived elsewhere and brought their anti-Semitic views to one of the country’s most diverse cities.

“It’s prevalent everywhere, sadly, and it can happen anywhere,” the mayor said of anti-Semitism.

There’s now a small, granite memorial at the spot where Seals died, which reads: “Loving son, Father, Brother and Friend to All.”

Family, friends and members of law enforcement will gather at that spot Thursday, on the one-year anniversary of the terrorist attack, to remember Seals. Also, at 10 a.m., state and federal officials will convene on Zoom to discuss how New Jersey is combating bias and hate. Attorney General Gurbir S. Grewal will honor those lives lost and lead the discussion.