NEW YORK — U.S. authorities charged as many as 18 people with weapons trafficking, including for an alleged scheme to smuggle grenade launchers, shoulder-fired missiles and other Russian military weapons into the country.

The arrests resulted from a yearlong investigation that used a confidential informant posing as an arms trafficker selling weapons to terrorists, according to prosecutors, the FBI and police.

The informant contacted the FBI after he was approached by a man who said he had access to weapons from the former Soviet Union and believed the informant, an explosives expert, could find a willing buyer, according to a law enforcement source.

Over the following year, the informant purchased eight assault weapons in locations around the country. Using a digital camera provided by the informant, members of the ring, which included Armenians and South Africans, provided pictures of the weapons they said they had available for sale, the official said.

The pictures, apparently taken somewhere in Armenia, show anti-tank missiles, a Russian missile launcher and a recoilless anti-tank rifle, among other weapons, the official said.

Arrests have been made in New York, Miami and Los Angeles, the source said.

The defendants also are charged in a criminal complaint with conspiring to traffic in machine guns and other assault weapons, and with selling eight such weapons.

Details of the investigation were to be discussed at a news conference later in the day, the office of U.S. Attorney David N. Kelley said.