Eight suspects from Tajikistan with suspected ties to ISIS, who crossed into the United States from the southern border last year and this year, have been arrested in Los Angeles, Philadelphia and New York City, according to a source familiar with the investigation.

The suspects were initially allowed to enter the U.S. after being vetted and no national security issues were uncovered, the source said.

Later, and in recent weeks, authorities uncovered derogatory information indicating ties or affiliation with ISIS and the suspects were sought and arrested.

Efforts are underway to deport the suspects as currently authorities have not developed enough evidence to bring any terrorism charges.

"Over the last few days, ICE agents arrested several non-citizens pursuant to immigration authorities," the FBI and the Department of Homeland Security said in a statement.

"The actions were carried out in close coordination with the FBI's Joint Terrorism Task Forces. The individuals arrested are detained in ICE custody pending removal proceedings.

"As the FBI and DHS have recently described in public and partner bulletins, the U.S. has been in a heightened threat environment. The FBI and DHS will continue working around the clock with our partners to identify, investigate, and disrupt potential threats to national security," the statement said.