WASHINGTON (CNN) -- Federal investigators are looking overseas for clues and suspects in a scam that uses bomb threats to extort money from banks and stores, law enforcement officials told CNN.
Wal-Mart employees gather outside the store in Newport, Rhode Island, after a bomb threat on Tuesday.
For the past week, banks and stores in at least 13 states have been hit by the scam, in which a caller claims there is a bomb on the premises that can be detonated if employees don't meet a demand to wire money to a specific account.
In one incident in Kansas, the caller ordered store employees and customers to take off their clothes, police said.
A source told CNN that investigators are looking for a suspect in Portugal who appears to be linked to an account number the caller uses in his demands. The source said it is believed to be either a single person or a small group tied to an account in Portugal. Watch SWAT teams respond to bomb threats »
At least $13,000 has been extorted, according to a law enforcement source.
One of the latest calls came into the Hannaford grocery store in the small town of Millinocket, Maine, on Wednesday.
As in many of the cases, the caller "[made] it seem very realistic that they're right in the building or right outside, almost like they have a visual," Millinocket Police Chief Donald Bolduc said.
About 38 shoppers and employees stayed in the store for at least three hours while police investigated the call, the chief said.
According to the Bangor Daily News, employee Linda Day answered the phone and the caller said, "This is a threat. Don't put me on hold."
The call startled Day, who said "What?" and then handed the phone to someone else, who verified the threat, the newspaper reported.
Bolduc said police tried to trace the call, which originated from a cell phone. "It was either national or international ... it was not a local cell phone company," he said.
Bolduc said the scare was "quite significant" to the small rural community, about 72 miles north of Bangor.
"You just don't think of things happening like this in our area," he said.
Bolduc said his department and other local law enforcement agencies are working with the FBI.
FBI spokesman Rich Kolko said the spate of scam calls started in Portland, Oregon, on August 23 and picked up momentum three days later.
"The investigation and leads so far point to it being likely this is one person or one group," he said. "This is criminal."
Kolko added there is no link to terrorism.
In addition to Maine and Oregon, the scam calls have been received in Arizona, California, Connecticut, Kansas, Michigan, Missouri, Pennsylvania, Rhode Island, Texas, Utah and Virginia, officials said.
The FBI is investigating an incident at a Dillons grocery store in Hutchinson, Kansas, and one at the Nodaway bank in Savannah, Missouri, which received a threat Friday, Kansas City FBI spokesman Jeff Lanza told CNN.
He said both attempts succeeded, but would not disclose how much money was involved.
"We're looking at a foreign connection," he said.
Lt. Paul Scofield of the Hutchinson police told CNN that several other bomb threats in the town on Wednesday were determined to be copycat crimes.
On Tuesday, more than 100 employees and shoppers at a Dillons were ordered to take off their clothes, and led to believe the caller was watching them, Scofield said.
"Some did actually disrobe, and others didn't," he said.
Police are still investigating whether the caller was watching the store, he said, adding that "whoever it was sure had them convinced they were."
In another incident, a Wal-Mart store in Newport, Rhode Island, was evacuated Tuesday morning after getting such a bomb threat.
The caller to the Wal-Mart demanded that $10,000 be wired to a location outside the United States, according to Sgt. James Quinn with the Newport police. The frightened employees did not send the full $10,000, a federal law enforcement source said.
The employees were afraid the would-be bomber was in their store; they didn't leave until a SWAT team arrived, Quinn said.
Quinn said early speculation that the caller was a disgruntled employee was shot down and that the call was traced to outside the United States. He would not disclose the location. E-mail to a friend
CNN's Carol Cratty, Jeanne Meserve and Katy Byron contributed to this report.
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