NEWPORT, R.I. – Large grocery and discount stores across the country have been targeted by a caller who threatens to blow up shoppers and workers with a bomb if employees fail to wire money to an account overseas, authorities said.

Frightened workers have wired thousands of dollars – and in one case took off their clothes – to placate a caller who said he was watching them but may have been thousands of miles away. The FBI and police said Wednesday they were investigating similar bomb threats at more than 15 stores in at least 11 states – all in the past week.

“At this point, there’s enough similarities that we think it’s potentially one person or one group,” FBI spokesman Rich Kolko said from Washington.

No one has been arrested, no bombs have been found, and no one has been hurt, though the calls have triggered store evacuations and prompted lengthy sweeps by police and bomb squads.

Law enforcement officials say the caller claims to have a bomb and orders the store to send money to an account through an in-store money transfer service such as Western Union. He often claims to be able to see inside the store, but officials believe he is making it up.

In Newport, employees at a Wal-Mart got three calls Tuesday morning and wired three payments totaling $10,000 to an account out of the country, Sgt. James Quinn said. A spokeswoman for Wal-Mart Stores Inc. said the company was assisting in the investigation, but offered no further comment.

The first of the threats that federal investigators are aware of came last Thursday at a Safeway in Sandy, Ore. The caller initially said he had a gun and was watching the store, but after meeting resistance to his demands he claimed to have a bomb, Sandy Police Chief Harold Skelton said.

In Buchanan, Mich., on Monday, the caller directed employees of a Harding’s market to lock the front doors, move to the front and told them not to call police, said Berrien County Sheriff Paul Bailey. The man claimed he could see some workers standing up and ordered them to sit down.

“He’s just ad-libbing,” Bailey said. “He can’t see anything.”

Nonetheless, Bailey said, the employees were so afraid they wired the caller $3,000. The manager even hung up the phone when authorities called, saying a bomb would go off if he talked to them.

Bailey said that in a phone call with police, the man even offered to trade a “hostage” for a police officer to make his threat more believable.

The caller has not gotten every store he has called to give up money, but the FBI on Wednesday did not provide the total amount taken.

On Tuesday, at a Dillons grocery store in Hutchinson, Kan., the caller ordered customers and employees to disrobe. Employee Marilyn Case told The Hutchinson News that store manager Mike Piros argued with the caller, but they relented when he continued to make threats and instructed them to “do it now.”

He then demanded that one of Piros’ fingers be cut off for every hour his demands were not met, and another employee got a butcher knife on his orders, Case said. Jim Peterson, a customer, told the newspaper that people became distraught.

“People came undone and started saying, ‘No, no,”‘ he said.

Piros was not harmed. Police there initially said they were investigating whether the caller had hacked into the surveillance system, but later backed away from that possibility.

Kapin said the FBI found the call was made from a cell phone registered to a Los Angeles phone number but was leased out from a European company. Investigators determined the call had come from somewhere in Portugal.

Callers also tried to extort money with calls to a US Bank in Boise, Idaho, on Wednesday morning; a Wal-Mart in Hutchinson, Kan.; bank branches at Wal-Marts in Salem, Va., and Fairlawn, Va., on Tuesday; to a Vons store in Vista, Calif., near San Diego, on Friday; and to two Giant Eagle grocery stores in the Pittsburgh area, authorities said. The FBI said it was also investigating similar incidents at a grocery store in Orem, Utah, on Monday and a store in McAllen, Texas, on Saturday.