Anchored by Jake Tapper, The Lead airs at 4 p.m. ET on CNN.
Continuing coverage of Malaysia Airlines Flight 17. Plus, the latest on Mideast tensions.
Members of the al Qaeda-linked terror group Al-Shabaab killed at least 62 civilians at an upscale mall in Nairobi, Kenya over the weekend. Security forces were still sweeping through the Westgate Mall searching for attackers Monday evening, and will likely continue through Tuesday.
Before its Twitter account was suspended, Al-Shabaab issued a list of the names of nine people it said were among the attackers, saying three were from the United States.
Al-Shabaab has claimed in the past that up to 50 Americans are members of the terrorist group. Although Al-Shabaab's focus is regional, there is a concern that the group could launch an attack in the U.S., said Republican Michael McCaul, chair of the House Homeland Security Committee.
"When you have 50 Americans, up to 50 Americans, trained to fight the war in terrorism, the idea that they could come back to the United States is a real valid concern," McCaul said in an interview with "The Lead with Jake Tapper."
The style of attack unleashed in Kenya "is one that we've had great concern with as well, because a soft target like a shopping mall is a very easy target to hit," said McCaul.
Americans who have been trained abroad in warfare, or copy cats "trying to perpetrate this type of scenario in the U.S. is of grave concern," said the Texas congressman.
Homeland Security and the FBI track Americans who travel abroad to join and train with groups like Al-Shabaab. But asked if he is confident that those Americans would immediately be caught if they tried to return to the U.S., McCaul replied, "I'm concerned."
"We have a pretty high degree of confidence as to who they are, but there possibly could be Americans over there that we don't know about. And I think that's one of my biggest concerns," said McCaul.
"They are going over there to fight the fight for Somalia, for the Al Shabab movement. But the idea of them being – coming back into the United States is a very real threat that we have to prepare ourselves for," said McCaul.
Navy Yard shootings
The horrific Navy Yard shootings happened just one week ago. Two of McCaul's colleagues, Reps. Henry Waxman and Anna Eshoo, sent a letter to the FTC and the Commerce Department asking about reports that first responders at the Navy Yard had faulty radios, and had to rely on personal phones to communicate.
Meanwhile, the police union is calling on Congress to investigate whether a tactical team from the U.S. Capitol Police was turned away from responding to the shooting.
McCaul said he planned to hold hearings about what he called the "inexcusable" lack of communication between agencies.
He said a SWAT team member outside the classified briefing room at the Capitol flagged yet another incidence of crossed signals.
"He told me about how – and I heard this firsthand – that his commanding SWAT team officer told them to stand down, that they were there eating breakfast, and that he told them to stand down," said McCaul.
"That's obviously of great concern when you have a SWAT team that close by that could respond to the situation," McCaul said.