Stay vigilant, shoppers.

Police uncovered credit card skimmers installed in a Market Basket and a Walmart in Concord, N.H., this week and are working with police in a handful of other Massachusetts and New Hampshire towns to see if skimmers found in stores there were the work of the same people or crew.

“Be vigilant, especially now going into the holiday season,” Concord Police Deputy Chief John Thomas told the Herald, adding that this is the season when police generally see an uptick in credit card information theft.

A credit card skimmer, which his department said it found in October in a self-checkout lane at the Loudon Road Walmart and the Storrs Street Market Basket, is a device that overlays and looks nearly identical to the legitimate credit card machines found in stores, at the gas pump, ATMs and anywhere else you’d find them.

“At this time, we are not aware of any customer information being compromised and we will continue to monitor this situation closely,” Justine Griffin, a spokeswoman for Tewksbury-based Market Basket, wrote in a statement. “We are also working with both state and local authorities to help identify the responsible individuals.”

Concord Police said a “telltale sign with these particular ‘skimmers’ is that the card chip reader slot is inoperable and appears ‘jammed,’ causing the customer to swipe the card so that the magnetic reader can steal the card information.”

“It’s very easy to install them, they just snap over the old credit card machines,” Thomas said, adding that if a customer has any trouble using the machine as normal to try a little tug on the top to see if a skimmer has been placed. “You can see how easy it is to steal some of this information.”

Thomas said that his department has connected with investigators in the local police departments of Nashua, N.H., and departments in Somerville, Reading, and Haverhill — all places where the scam devices have also recently cropped up — to look into what may be a serial credit card skimming operation.

Concord Police had determined that the machines at the Walmart and the Market Basket were installed by the same two men, images of whom were captured by store surveillance cameras and released by authorities, wearing scally caps, long coats and surgical masks of the kind popular during the pandemic. He said the suspects in the other towns wore similar outfits, but the masks and outfits make it hard for investigators to determine if they are the same perpetrators.

While Thomas said that people have called in reports of credit card fraud since these incidents were reported, he said it’s hard to determine if they were compromised at these stores or elsewhere.

The federal Office of the Comptroller of the Currency also warns of higher-tech skimmers that can read the chips in cards combined with the placement of a camera to read pin numbers. The office recommends walking away and speaking with a bank representative or a law enforcement officer if anything seems off, and to take precautions like checking for places a camera could be concealed and for plastic sheaths inserted into the card slot.

The Federal Trade Commission reports that fraud in general continues to trend upward and ate up $8.8 billion from Americans in 2022, losses 30% greater than the year before.

A Walmart corporate spokesman told the Herald that “this situation remains an on-going criminal investigation, and we’re actively engaged with law enforcement.”

“Providing customers with a safe shopping experience is a top priority. We’re continually reviewing protocols and adding enhanced security measures to better protect in-store transactions,” Joe Pennington, the director of Walmart’s global communications office, wrote in a statement, adding that any customers concerned that they may have been impacted should call their card provider or Walmart’s customer care team at 1-800-925-6278.

Concord Police asks that anyone with any information on these incidents or the suspects pictured to contact Det. Stephen Hemming by emailing shemming@concordpolice.com or calling 603-225-8600, extension 4854. People living in other jurisdictions should contact their local police.

One of the credit card skimmer devices seized at either a Walmart or Market Basket store in Concord, N.H. The device, whose top, visible portion is seen at left and its reverse at right, is laid over the legitimate credit card machine and records personal financial data off of a swiped credit card. (Collage, Flint McColgan / Boston Herald, composed of courtesy photos from Concord PD)