(CNN) — US federal agents have arrested eight Tajikistan nationals located in the United States on immigration charges following the discovery of potential ties to terrorism, two sources familiar with the law enforcement operation told CNN Tuesday.

The arrests by US Immigration and Customs Enforcement – first reported by the New York Post – included apprehensions in Los Angeles, New York and Philadelphia, the sources said.

One source said the eight had previously entered the US via the Southern border and were screened by US officials, and that no derogatory information in their past was identified at the time.

A second source said investigators later discovered possible links to ISIS members located overseas, which spurred the federal investigation. The method of identifying the suspects inside the US was accomplished in part by the US government’s highly sensitive targeting of the communications of ISIS members abroad, the source said.

The group had been on the radar of US officials for well over a month, but senior US officials recently decided to have the eight expelled from the country under ICE’s Enforcement and Removal Operations authority rather than risking having the FBI surveil them longer and wait for the potential manifestation of a possible plot, the source told CNN.

The ability to surveil certain foreign targets was the subject of intense debate in Congress earlier this year, with conservatives criticizing the US intelligence community’s sweeping powers. A two-year authorization of one particular aspect of the Foreign Intelligence Surveillance Act – known as Section 702 – finally passed both houses of Congress and was signed by the president in April before the program lapsed.

In a release Tuesday, ICE and the FBI said ICE “agents arrested several non-citizens” in coordination with the FBI’s joint terrorism task forces.

“The individuals arrested are detained in ICE custody pending removal proceedings” the release reads. “As the FBI and DHS have recently described in public and partner bulletins, the US has been in a heightened threat environment.”

Investigators do not currently believe the eight received training abroad or were purposefully sent to the US to engage in violence, one source said.

Of those arrested, a small subset is believed to have espoused concerning extremist rhetoric, according to the source, and it is unclear whether the remaining people were arrested for their mere association with the other people arrested.

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