TEHRAN, Iran ( AP) — Thousands of mourners chanted “Death to Israel” and “ Death to America” today during the funeral of a slain nuclear expert whom Iranian officials accuse the two nations of killing in a bomb blast this week as part of a secret operation to stop Iran’s nuclear program.

The assassination of Mostafa Ahmadi Roshan has raised calls in Iran for retaliation against the U. S. and Israel, and an independent news website today said Iran is preparing a covert counteroffensive against the West.

Roshan, a chemistry expert and a director of the Natanz uranium enrichment facility in central Iran, was killed in a brazen daylight assassination when two assailants on a motorcycle attached a magnetic bomb to his car Wednesday in Tehran. The killing bore a strong resemblance to earlier killings of scientists working on the Iranian nuclear program.

State TV showed thousands of people carrying Roshan’s coffin through central Tehran before it was taken to a north cemetery for burial.

Iran’s Supreme Leader Ayatollah Ali Khamenei, who has the final say on all state matters, accused the U. S. and Israel of being behind the attack. He vowed Thursday that the perpetrators and those who ordered the attack would be punished.