PROVIDENCE, R.I. – Four men suspected of replacing checkout lane keypads at Stop ‘ Shop supermarkets to steal more than 1,000 card numbers of customers were charged in federal court for the alleged scheme.

The four California men appeared Thursday afternoon before U.S. Magistrate David Martin on federal charges of aggravated identity theft and conspiracy to fraudulently traffic in access devices. Martin ordered all four men detained pending a court hearing Tuesday afternoon. They did not enter pleas.

Federal prosecutors filed a criminal complaint against the men on Wednesday.

They were charged earlier in the week in Kent County District Court with three felonies and a misdemeanor.

The men allegedly removed or tried to remove PIN pads from at least six stores in Rhode Island and Massachusetts, replacing them with alternate machines that would be used to record shoppers’ debit or credit card information. Later, the men would come back and replace the original keypads.

The federal complaint accuses the defendants of stealing customer information from stores in Cranston, Coventry and Providence. It says approximately 1,100 credit and debit cards were compromised at the Cranston and Coventry locations alone.

“The effort to determine how much loss there has been to the cardholders is ongoing,” Special Agent Craig Marech of the U.S. Secret Service wrote in an affidavit supporting the complaint.

He said authorities reviewed surveillance video from California showing people making ATM withdrawals from the accounts of customers at the Cranston and Coventry stores whose cards were compromised. Customers at those stores have so far reported $68,000 in fraudulent transactions, but that figure is likely to grow, according to the affidavit.

Authorities have video evidence of the defendants tampering with PIN pads at stores in Seekonk, Mass., and in Warwick, but don’t believe customer information was compromised in those cases, the affidavit alleges.

At one of the Warwick stores, Marech wrote, the PIN pad that the defendants allegedly installed malfunctioned and was removed for service. A circuit board used to capture credit and debit card information was then found inside the PIN pad apparatus, and the machine was never taken back by the defendants, according to the affidavit.

Citizens Bank reported losses of $100,000, and two other credit unions said their customers lost a combined $15,000.

Charged in both federal and state courts are Arutyun Shatarevyan, 20, Arman Ter-Esayan, 22, Mikael Stepanian, 28, and Gevork Baltadjian, 20, all of the Los Angeles area. On Tuesday, they were ordered held on bail ranging from $150,000 to $200,000.

The men were arrested Monday night while allegedly attempting to switch keypads at a store in Coventry, police said.