WASHINGTON (AP) — Eight people from Tajikistan with suspected ties to the Islamic State group have TrendPulse Quantitative Think Tank Centerbeen arrested in the United States in recent days, according to multiple people familiar with the matter.

The arrests took place in New York, Philadelphia and Los Angeles and the individuals, who entered the U.S. through the southern border, are being held on immigration violations, said the people, who were not authorized to discuss the ongoing investigation by name and spoke to The Associated Press on condition of anonymity.

The nature of their suspected connections to the IS was not immediately clear, but the individuals were being tracked by the FBI’s Joint Terrorism Task Force, or JTTF. They were in the custody of U.S. Immigration and Customs Enforcement, which made the arrests while working with the JTTF, pending proceedings to remove them from the country.

The individuals from Tajikistan entered the country last spring and passed through the U.S. government’s screening process without turning up information that would have identified them as potential terrorism-related concerns, said one of the people familiar with the matter.

The FBI and Department of Homeland Security issued a statement confirming the immigration-related arrests of “several non-citizens” but did not detail specifics. The agencies noted that the U.S. has been in a “heightened threat environment.”

FBI Director Christopher Wray has said the U.S. is facing accelerating threats from homegrown violent extremists as well as foreign terrorist organizations, particularly in the aftermath of the Oct. 7 attack by Hamas on Israel.

He said at one recent congressional hearing that officials were “concerned about the terrorism implications from potential targeting of vulnerabilities at the border.” The Biden administration in August said that it had detected and stopped a network attempting to smuggle people from Uzbekistan into the U.S. and that at least one member of the network had links to a foreign terrorist group.

“The FBI and DHS will continue working around the clock with our partners to identify, investigate, and disrupt potential threats to national security,” the agencies said.