In what could prove to be one of the biggest counterterrorism seizures in U.S. history, prosecutors filed a civil complaint in federal court seeking the forfeiture of more than $500 million in assets of the Alavi Foundation and an alleged front company.

The assets include Islamic centers in New York City, Maryland, California, and Houston; more than 100 acres in Virginia; and a 36-story office tower in New York.

Iran's U.N. Mission did not immediately respond to a call and e-mail seeking comment.

It is extremely rare for U.S. law enforcement authorities to seize a house of worship, a step fraught with questions about the First Amendment right to freedom of religion.

The action against the Shiite Muslim mosques is sure to inflame relations between the U.S. government and American Muslims, many of whom are fearful of a backlash after last week's Fort Hood shooting rampage, blamed on a Muslim American soldier.

The mosques and the office tower will remain open while the forfeiture case works its way through court in what could be a long process.

There were no raids yesterday as part of the forfeiture action. The government simply posted notices of the civil complaint on the property.

Prosecutors said the Alavi Foundation, through a front company, Assa Corp., illegally funneled millions in rental income back to Iran's state-owned Bank Melli.

Bank Melli has been accused by a U.S. Treasury official of providing support for Iran's nuclear program, and it is illegal in the United States to do business with the bank.

"For two decades, the Alavi Foundation's affairs have been directed by various Iranian officials, including Iranian ambassadors to the United Nations, in violation of a series of American laws," U.S. Attorney Preet Bharara said in a statement.

The skyscraper, known as the Piaget building, was erected in the 1970s under the shah of Iran, who was overthrown in 1979. The tenants include law and investment firms and other businesses.

The sleek building, valued at $570 million to $650 million in 2007, has been an important source of income for the foundation for 36 years. The most recent tax records show the foundation earned $4.5 million from rents in 2007.

The rents help fund the centers and other ventures, such as sending educational literature to Muslims imprisoned in the United States. The foundation has also invested in dozens of mosques around the country and supported Iranian academics at universities.