ABC News' Pierre Thomas, Mike Levine, Jack Date, and Jack Cloherty report:

Federal authorities are worried that Americans or other Westerners who have trained with terrorists in war-ravaged Syria may slip back into the United States looking to strike the U.S. homeland, Homeland Security Secretary Jeh Johnson told ABC News in an exclusive interview.

"We're very concerned about Syria foreign fighters, people who are going into Syria, who are being recruited by extremists there and who then may leave Syria with a different purpose in mind," Johnson told ABC News' Pierre Thomas for "This Week," in his first network interview since becoming Homeland Security secretary in December 2013.

Several Americans have already been arrested inside the United States for Al Qaeda in the Arabian Peninsula - the Yemen-based affiliate behind the failed Christmas Day "underwear bombing" in 2009 and a cargo-based plot the next year - is "still very active."

"They're still making efforts to attack the homeland," he said.

Johnson, who previously served as general counsel in the Department of Defense, now leads an agency borne out of the Sept. 11, 2001, attacks - a nightmare he experienced firsthand in an intensely personal way.

Not only is Sept. 11 his birthday, but he was in New York that tragic day watching it all unfold in front of him from his law office in Manhattan.

"I was looking forward to a quiet birthday dinner with my family that evening," Johnson recalled. "And I remember looking out the window, and I could see the World Trade Center on fire. And within minutes, I saw the second plane hit… And [then] I saw the buildings collapse."

Johnson left his office building "and literally wandered the streets asking, 'What can I do?'" he told ABC News. "And so since that day, I've tried to answer the question, 'What more can I do?' And so I'm here in the Department of Homeland Security. I didn't expect to be here, but I'm here and I believe very much in the mission of homeland security."

But to accomplish that mission, Johnson said he needs to address a serious issue within his own agency: employee morale.

That's why on Friday, he sent a message to his entire workforce of 240,000 men and women.

"Let us not forget that the Department was founded in reaction to the most horrendous terrorist attack in our nation's history and that more than a decade later, threats to our homeland security remain real," Johnson said in the message. "There is no room for complacency. I need your continued dedication, I need your help."

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