DENVER — Immigration agents operating near Vail placed branded ace of spades playing cards — similar to “death cards” left on corpses by U.S. forces during the Vietnam War — in cars left behind after immigrants’ arrests this week, an advocacy group alleged.

U.S. Immigration and Customs Enforcement is investigating, according to a statement issued Friday afternoon.

The cards were stamped with “ICE Denver field office” and the address and phone number for the immigration detention center in Aurora, said Alex Sánchez, the president and CEO of Voces Unidas, a Glenwood Springs-based immigrant-rights advocacy group. The cards were left in several cars abandoned after their occupants were arrested by ICE agents this week in Eagle County, during what Sánchez described as fake traffic stops.

Sánchez said eight people were arrested in the traffic operation and his group has confirmed they’re detained in the Aurora facility. Avon police Chief Greg Daly told the Vail Daily that ICE had confirmed seven arrests and detentions carried out Wednesday morning near Eagle-Vail.

Sánchez said family members who came to retrieve the abandoned vehicles found the playing cards. Some of the cars, he said, were left running in the middle of a highway west of Vail, and their drivers had pulled over thinking ICE agents were local police conducting traffic stops.

He said Voces Unidas staff had also seen the cards and had spoken with family members who’d found them.

In a statement, an unnamed spokesperson for the U.S. Department of Homeland Security said ICE “unequivocally” condemned “this type of action and/or officer conduct.” After learning about the cards, supervisors quickly stepped in “to address the issue.”

“The ICE Office of Professional Responsibility will conduct a thorough investigation and will take appropriate and swift action,” the spokesperson wrote.

The cards are similar to those left by some American troops on the bodies of Vietnamese soldiers during the Vietnam War, a “psychological warfare” practice described by the president of the U.S. Playing Card Co. in a 1966 letter to a congressman.

“It is a racist symbol, and it’s just disgusting to see the actions of ICE in our community,” Sánchez said. “Leaving a racist death card behind for families to find is just sickening.”

Rumors of ICE activity in the Vail area Wednesday prompted some businesses to shut down early and at least two restaurants to close for the day, the Vail Daily reported.

Denver immigration attorney Hans Meyer told The Denver Post he had spent Friday consulting with the men arrested in the Eagle County traffic operation. He said he was “investigating the potential unlawfulness of ICE’s actions.”

State Rep. Elizabeth Velasco, a Democrat, said the cards were an “intimidation tactic.”

“It’s disgusting that ICE is using these racist tactics to intimidate our communities,” she said.