(CNN) - The Islamic State terror group is now "a credible alternative to al Qaeda" that is "expanding its presence" with foreign fighters returning from Syria, and possibly Iraq, to their home countries, a U.S. intelligence official said Thursday.
The official, who declined to be identified due to the sensitive nature of the information, has direct knowledge of the latest intelligence on the group, also known as ISIS or ISIL.
According to an assessment, the group has grown in size since the spring and its takeover of Mosul in northern Iraq as more fighters from around the world have mainly traveled to Syria to join its ranks.
The United States believes that while the group remains largely focused on its brutal takeover of large areas of Iraq, there is also an "expansion of its external terrorist ambitions."
United States officials are tying the new threat of shoe bombs on airplanes to al Qaeda.
There are indications that the threat could ultimately be tied to Ibrahim al Asiri, the master bomb-maker for al Qaeda in the Arabian Peninsula.
He's the man officials say designed the underwear bomb that failed to detonate aboard a Detroit bound flight on Christmas Day in 2009.
He also built the printer-cartridge bombs that al Qaeda placed on cargo planes destined for Chicago the following year.
Al Qaeda link to shoe-bomb warning
An official tells CNN the new warning is related to recent intelligence gathered on bomb-making tactics believed tied to Asiri.
CNN's Erin Burnett talks to National Security Analyst and Former CIA operative Bob Baer about the latest threat and why Asiri hasn't been caught.
"He's up in the mountains of Yemen," Baer says. "There's no digital footprint that he's left behind. He's read about leaks in this country. He's fairly immune and there's nothing the Yemeni government could do about it."
Is the U.S. trying to make a deal with the Taliban?
There is word the United States is trying to negotiate the release of Army Sgt. Bowe Bergdahl, who has been held captive by a Taliban-affiliated group for nearly five years. He is the only know U.S. prisoner in Afghanistan.
In exchange for Bergdahl, the U.S. would reportedly free five senior Taliban prisoners held at Guantanamo Bay.
For Bergdahl's family in rural Idaho, the wait is excruciating.
OutFront: Gunnery SGT. Jessie Jane Duff, a 20-year veteran of the Marine Corps who provided logistical support to combat units in Iraq and Afghanistan.
More than a year after four Americans were killed in a terrorist attack on the U.S. consulate in Benghazi, no one has been brought to justice.
Now a new investigation by the New York Times is raising even more questions.
According to the report, there is "no evidence that al Qaeda or other international terrorist groups had any role in the assault…" and it “…was fueled in large part by anger at an American-made video denigrating Islam."
This contradicts what many critics of the administration have been saying for months.
Republican Congressman Jason Chaffetz is one of those critics.
"I hope and pray to God that you, as leader of the United States, along with your administration, will feel an adequate level of responsibility toward me and work for my release."
An American captured by Al Qaeda in Pakistan two years ago says he feels abandoned and forgotten by the U.S.
In a new video released on Christmas day, Warren Weinstein pleads with President Barack Obama to negotiate for his release - a practice the U.S. says it does not engage in.
Jill Dougherty is at the State Department with all the details.