On Tuesday, The Daily Beast reported that there are more than 100 Americans fighting on behalf of Al Qaeda-linked groups in Syria. Not only that, the jihadist Americans were apparently making their way back to the United States uninhibited.

A senior US official told The Daily Beast that somewhere between six and twelve American-born Islamist radicals are back in the United States. The senior official was quoted as saying, “We know where some are,” which may not be too comforting given their recent activities.

The amount of Americans fighting abroad for Islamist causes has increased exponentially over the past year. Last fall, an estimated ten jihadi Americans were fighting with the Syrian Islamist rebels against the Assad regime. Fast forward to January and that number increased to 70 individuals, according to The New York Times.

In March, National Counter-terrorism Center director Matthew Olsen spoke of the recent surge of foreign fighters serving in Syria. “In light of the large foreign fighter component in Syria crisis, we are working together to gather every piece of information we can about the identity of these individuals,” he said.

A senior US official told The Daily Beast the NSA doesn’t have the capacity to track the constant flow of westerners in and out of Syria. “The NSA does not have the ability to track thousands of bad guys–and on the human intelligence side, this is even more difficult, so we are worried that people are slipping through the cracks,” said the official.

Officials and experts also worried that foreign fighters were being trained overseas in order to carry out attacks against the United States. The case of Faisal al-Shazad shines light upon this worry. Shazad was a Pakistani-American who became radicalized overseas and then attempted to detonate a bomb in Times Square, New York City, in 2010.