Three men face federal charges in connection with a drug ring that used tour buses to move millions of dollars and marijuana between Detroit and Tucson, Ariz.

U.S. Attorney Steven Biskupic said authorities have seized about $3.7 million in cash from the drug ring in recent weeks, according to a statement issued by his office in Milwaukee late Friday afternoon.

Terrance Lamont Davis, of Tucson, and Calvin C. Wiggins and James L. Reaves, both of Detroit, have been charged with conspiracy to distribute more than 1,000 kilograms of marijuana. The three men are being held in Tucson. They could face life in prison and up to $4 million in fines if convicted.

Biskupic's statement said the Internal Revenue Service's criminal investigation division, the Wisconsin Department of Justice, the Illinois and Michigan state police, the Arizona Department of Public Safety and the Oklahoma State Highway Patrol and Oklahoma County Sheriff's Department and Oklahoma City Police all participated in the bust.

A criminal complaint filed in federal court in Milwaukee said the ring used a Chicago-based tour bus company called Jackson Coach Lines as a front. The organization had two buses and paid people to act as passengers and drivers.

The buses would travel from Chicago to Detroit, where they were loaded with large bags full of drug money. A confidential informant told a Wisconsin Justice Department agent he had ridden on three separate trips between November 2006 and January 2007, the complaint said.

Passengers were told to lay down and not look out the window during the transfers, the informant said. Another confidential informant said he served as a driver and saw multiple packages being loaded onto the bus.

The buses would then travel to Arizona to pick up marijuana. They would stop first in Tucson, where the money would be taken from the buses and loaded into a waiting vehicle.

The buses would then continue on to Phoenix, where they'd spend the night. The next day they would travel to Tucson again and fill the luggage compartments with marijuana.

They would drop the drugs in Detroit before returning to Chicago.

On Monday, law enforcement agents followed one of the two buses from Chicago to Detroit. The bus then left Detroit for Arizona. Police watched bags transferred from the bus' luggage compartments into a car at a restaurant.

The bus then traveled to Phoenix while the car went to a Tucson-area house. Agents executed a search warrant there on Wednesday morning and captured $1.4 million in cash from the bags.

Arizona Department of Public Safety Sgt. Mark Morlock said the 32-year-old Davis was arrested as he sped away from the house, while the 26-year-old Wiggins was captured in the house. Police arrested Reaves, 20, at another location in Tucson later that afternoon, Morlock said.

Meanwhile, another group of police were tailing the second bus, which had left for Detroit. They pulled that bus over near Oklahoma City on Wednesday. Police found about $1.2 million in cash in its luggage departments.

Police pulled over the first bus in Tucson shortly after the second bus was stopped in Oklahoma, the complaint said.

About 15 people were on the Tucson bus, the complaint said. One of them told agents the two buses would communicate with each other. Biskupic's statement does not say what became of those people.

The Oklahoma bus was carrying about 17 people, including three to five Wisconsin residents and a man agents believe is one of the ring's leaders. He has not been charged. Biskupic's statement does not say what became of those people, either.

A message The Associated Press left at Biskupic's office Friday afternoon wasn't immediately returned.

Morlock said he didn't have information about how many people may have been on the bus in Arizona. He referred further questions to the Wisconsin Department of Justice, which did not immediately return a call seeking comment.

A recorded greeting with the Oklahoma Department of Public Safety didn't allow a caller to leave a message. A message left with the Oklahoma County Sheriff's Department wasn't returned.

The two confidential informants who spoke to Wisconsin justice agents told them they robbed one of the buses in January and made off with $1.3 million. Police in Milwaukee recovered about $1.1 million of that money in March, the statement said. The two informants had spent the remainder on cars and a house in the Milwaukee area.