Johnson said he and other law enforcement and security officials around the world were focused on foreign fighters heading to the bloody war, including those from the United States, Canada and Europe.
In excerpts from his first major speech since taking office last year, Johnson did not discuss how many U.S. fighters may be in Syria, but he said the government is "very focused" on the problem."Based on our work and the work of our international partners, we know individuals from the U.S., Canada and Europe are traveling to Syria to fight in the conflict," he said.
U.S. intelligence officials have said a handful of Americans and hundreds of Europeans have already returned to their home countries. The officials spoke on condition of anonymity because they were not authorized to speak publicly on the topic.
The State Department has no estimates of how many Americans have gone to fight with Syrian rebels, but British defense consultant IHS Jane's puts it at a few dozen. An estimated 1,200 to 1,700 Europeans are among rebel forces in Syria, according to government estimates.
Director of National Intelligence James Clapper told the Senate Intelligence Committee last month that al-Qaida groups in Syria have started training camps "to train people to go back to their countries" - one of the newest threats emerging in the past year to U.S. security.
Clapper told senators that as many as 7,000 foreigners from some 50 countries, including Europe, were fighting with rebels and extremists in Syria.
To Johnson, it's not just people joining the fight in Syria that are a concern.
f"At the same time, extremists are actively trying to recruit Westerners, indoctrinate them and see them return to their home countries with an extremist mission," Johnson said. "Syria has become a matter of homeland security. DHS, the FBI and the intelligence community will continue to work closely to identify those foreign fighters that represent a threat to the homeland."
Much like his predecessor at Homeland Security, Johnson also said he is most concerned about "lone wolf" terrorists who haven't received any specific training from al-Qaida or other terror groups but instead have become self-radicalized.
"In many respects, this is the terrorist threat to the homeland - illustrated last year by the Boston Marathon bombing - that I worry about most," Johnson said. "It may be the hardest to detect, involves independent actors living within our midst, with easy access to things that, in the wrong hands, become tools for mass violence."
Johnson also touched on immigration his in prepared remarks, repeating earlier statements that immigration reform also is a matter of homeland security.
"We need the additional border and port security resources that immigration reform legislation would provide," he said.
But it's not just the border security concerns that make immigration reform a matter of homeland security, Johnson added.
"There are an estimated 11.5 million undocumented immigrants living in this country. They are not going
away. They are not going to 'self-deport.' Most have been here for years. Many came here as children," he said. "As
a matter of homeland security, we should encourage these people to come
out of the shadows of American society, pay taxes and fines, be held
accountable, and be
given the opportunity to get on a path to citizenship like others. This is not a special path to citizenship; it is an opportunity to get
in line behind those who are here legally. This is not rewarding people
for breaking the law; it is giving them the
opportunity to get right with the law. And it is far preferable to
what we have now."