The Coast Guard said Sunday that it has increased patrols across the Straits of Mackinac in response to a possible threat to the bridge, which connects Michigan's Upper and Lower peninsulas.
"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.
The men were arrested Friday outside a Wal-Mart Stores Inc. outlet in Caro, about 80 miles north of Detroit in the state's agricultural Thumb region, after employees became suspicious over their purchase of 80 cell phones.
Authorities say the men had about 1,000 cell phones in their van.
Maruan Awad Muhareb of Mesquite, Texas; Adham Abdelhamid Othman of Dallas; and Louai Abdelhamied Othman of Mesquite were charged with collecting or providing materials for terrorist acts and surveillance of a vulnerable target for terrorist purposes.
Tuscola County Prosecutor Mark E. Reene told reporters Saturday that investigators believe the men were targeting the Mackinac Bridge. He declined to say what led investigators to that belief.
But the Detroit Free Press, citing a law enforcement official familiar with the case that it didn't name, reported Monday that the phones plus photographs and videos of the bridge led authorities to believe the men were targeting the structure.
Authorities contacted the office of Gov. Jennifer Granholm about the men, The Detroit News reported. Granholm spokeswoman Liz Boyd said: "At this point, there is no credible evidence of a terrorist activity" against the bridge.
Adham Othman and Louai Othman are brothers and are in their early 20s. Muhareb, 18, is their cousin. They were being held on bonds of $750,000 each.
The men told a magistrate Saturday that they were buying the phones for resale.
Louai Othman's wife, Lina Odeh, told The Associated Press on Saturday that she thought her husband and her relatives were targeted because of their Arab descent. She said the men's families come from Jerusalem.
Authorities in Caro 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.
On Sunday, the FBI said it was assisting Michigan authorities in the investigation.
"Those individuals are being held on state charges," said Detroit FBI spokeswoman Dawn Clenney. "We're consulting with the local authorities there. We're monitoring this."
District Court Magistrate Joseph A. Van Auken set a pretrial hearing for Friday and a preliminary exam for Aug. 24.
Adham Othman said he was born in Jerusalem and has lived in Puerto Rico and, for the past 10 years, in Dallas. Louai Othman said he was born in Dallas, where he now lives, and has lived in Puerto Rico and Jerusalem.
Muhareb said he was born in Puerto Rico, attended high school in Jerusalem, and was living in Puerto Rico before moving to Dallas.