Tuscola County Prosecutor Mark E. Reene said investigators believe the men were targeting the Mackinac Bridge. He declined to say what led investigators to that belief.
"The additional patrols are being conducted to deter, detect and allow for a more timely response to any possible threats made to the bridge," the Coast Guard said in a statement Sunday.
The men were arrested Friday outside a Wal-Mart store in Caro, about 80 miles north of Detroit, after purchasing 80 cell phones. Authorities say the men had about 1,000 cell phones in their van.
The men said they planned to sell the phones for profit in Texas.
Maruan Awad Muhareb and Louai Abdelhamied Othman, of Mesquite, Texas and Adham Abdelhamid Othman of Dallas were charged with collecting or providing materials for terrorist acts and surveillance of a vulnerable target for terrorist purposes.
Authorities have not said what they believe the men intended to do with the phones, most of which were prepaid TracFones. But Caro's police chief said cell phones can be used as detonators, and prosecutors in a similar case in Ohio have said that TracFones are often used by terrorists because they are not traceable.
District Court Magistrate Joseph A. Van Auken set a pretrial hearing for Friday and a preliminary exam for Aug. 24.