U.S. warned Iran of Jan. 3 bombing attack, officials say.

Mourners gathered earlier this month in Tehran at the funeral a victim of a terrorist bombing in Kerman, Iran, on Jan. 3.

tktkt — The United States delivered a secret warning to Tehran ahead of a deadly bombing attack in Iran by the Islamic State on Jan. 3, in an attempt to help the Iranian government thwart the attack, according to American officials with knowledge of the plans.

The officials said the United States had intelligence indicating that an affiliate of the Islamic State, known as ISIS-Khorasan, was planning an attack in Kerman, Iran. The attack was to strike a public memorial service for Maj. Gen. Qassim Suleimani, an Iranian military officer who had been assassinated four years ago in an American drone strike, the officials said, speaking on the condition of anonymity to discuss sensitive intelligence.

On Jan. 3, 84 people were killed when a bomb exploded at the memorial service.

Immediately after the bombing, American officials said privately they believed the Islamic State had carried out the attack. A day later the group formally took responsibility.

Iranian officials at first blamed Israel and the United States for the bombing. Later, Iran launched missile strikes on Islamic State targets in Syria, an apparent retaliation for the attack.

U.S. intelligence agencies operate under a “duty to warn” rule. If an agency learns of intelligence that says a potential terror attack may occur, the U.S. government must deliver a warning.

It was not immediately clear if the information was collected by the United States or by an allied power, nor was it apparent whether Iran trusted the intelligence that the United States provided. The United States does not have diplomatic relations with Iran, so also remained unclear whether the warning was delivered to Iran through a back-channel communication, through another country, or by a different method entirely.

The warning was earlier reported by The Wall Street Journal.

The Islamic State is far weaker than it was a decade ago when its forces captured large swaths of Syria and Iraq. But the attack on Iran, as well as attacks in Afghanistan, shows that the group still has the capacity to conduct bombings.