Danish officials did not name the man; his group identified him as Habib Jabor, but that is an alias. He lives in Ringsted, a town southwest of Copenhagen, and has been under police protection since the spring because of a threat to his safety.

The Danish investigators received crucial assistance from the Mossad, which passed along information that it obtained in the course of its normal surveillance efforts, according to a high-ranking Israeli official, who asked not to be named when discussing sensitive matters.

Some Iranian officials initially blamed Arab separatists for the Sept. 22 attack on a military parade in the Iranian city of Ahvaz, which killed 25 people and wounded many more, while other officials blamed the Islamic State. When Iran fired missiles into Syria days later, in what it said was retaliation for the massacre, it identified the target as an Islamic State base.

An Arab separatist group, the Ahvaz National Resistance, took responsibility for Ahvaz attack, but Mr. Jabor’s group insisted that it was not involved. The two organizations have links, but the relationship between them is murky.

The Danish and Swedish police said that a Norwegian man of Iranian decent was arrested on Oct. 21 in Sweden and remains in solitary confinement, but has denied all charges.

In late September, the authorities noticed the man taking photographs of Mr. Jabor’s residence and “intended to pass on the information to an Iranian intelligence service with a view to the information forming part of the plans to assassinate the leader” the security service said.

A year ago another member of Mr. Jabor’s separatist group was gunned down outside his home in Amsterdam, and last month Belgium charged an Iranian diplomat over alleged plans to bomb a meeting of Iranian exiles.