Source Says US Investigating Deadly Kenya Terror Attack, as 'Most' Hostages Freed

Witnesses reported about five minutes of sustained,  heavy gunfire coming from the Westgate shopping complex shortly before 6:30 a.m. local time, a clear indication that at least one gunman was still free and that the standoff continued.  And hours after Kenyan officials said most hostages had been released, no further updates were released by authorities.

Meanwhile, the FBI and the U.S intelligence community are "aggressively" investigating whether or not Americans were among members of an Al Qaeda-linked militant group involved in an attack and hostage situation at a mall in Kenya, a federal law enforcement source told Fox News.

"Most" of the hostages who were holed up at mall were freed late Sunday after the country's military launched a major assault on the building where at least 68 people were killed and 175 injured the day before.

The military assault began shortly before sundown, with one helicopter skimming very close to the roof of the shopping complex as a loud explosion rang out, far larger than any previous grenade blast or gunfire volley.

"This will end tonight. Our forces will prevail. Kenyans are standing firm against aggression, and we will win," Kenya's National Disaster Operation Centre said on Twitter.

At around midnight local time, Kenya's Defence Forces said it had rescued most of the hostages and had taken control of most of the mall, but declined to give further information on those freed. Officials said four Kenyan military personnel were wounded in the operation.

Many of the rescued hostages -- mostly adults -- were suffering from dehydration, Col. Cyrus Oguna, a military spokesman, told The Associated Press. He refused to say how many hostages were rescued or how many were still being held. He said some of the attackers had "most probably" been killed in the operation.

On Saturday, the Islamists stormed the mall in an attack targeting non-Muslims that left at least 68 dead and 175 wounded.

The source tells Fox News that the U.S. intelligence community believes the attack is a "hot" and "major event," because it is outside the normal scope of al-Shabaab and the alleged "multi- national character" of the attackers.

New York Rep. Peter King, a member of the House Intelligence Committee, said on ABC’s “This Week” that al-Shabab is “one of the only Al Qaeda affiliates which actually has actively recruited here in the United States.”

He called the Saturday mall attack a "well coordinated, well planned massacre.”

Kenya’s president, Uhuru Kenyatta, said in a national address Sunday that security forces had managed to isolate the “criminals” in one place inside the building, according to Reuters. Two helicopters were seen flying over the mall.

There are between 50 to 200 hostages and most of them were hiding in various places inside the mall, Fox News confirms. They are not being held by the hostage-takers. There are between 10 to 15 militants currently inside the mall, with at least one being female.

Some of the people in hiding managed to escape before the military assault, according to Kenyan media reports.

Cecile Ndwiga said she had been hiding under a car in the basement parking garage.

"I called my husband to ask the soldiers to come and rescue me. Because I couldn't just walk out anyhow. The shootout was all over here -- left, right-- just gun shots," she said.

Police said that 49 people had been reported missing, according to a statement released by Kenya's Red Cross.

Kenyatta said in his address that the attackers "shall not get away with their despicable and beastly acts."

"We will punish the masterminds swiftly and indeed very painfully," he added.

The White House said Sunday that President Barack Obama called Kenyatta to tell him the United States supports his country's effort to bring al-Shabab to justice.

U.S. Secretary of State John Kerry condemned the attack as "an enormous offense against everybody's sense of right and wrong," and called the attackers "ruthless and completely reckless terrorists."

Kenyans and foreigners were among those confirmed dead, including French, Canadians and Chinese. The U.S. State Department said four American citizens were injured and were being given assistance. The age of the victims ranged from 2 to 78, Reuters reports.

Nineteen people, including at least four children, died after being admitted to Nairobi's MP Shah hospital, said Manoj Shah, the hospital's chairman. "We have at least two critical patients currently, one with bullets lodged near the spine," he said.

The hospital continued to receive patients Sunday, he said.

The militants assaulted the mall on Saturday and remained there throughout the night.

Sara Head, an American citizen who is in Nairobi for a business trip, told CBC that she was in the mall's parking garage when gunfire first broke out. She said she hid for an hour and a half inside a stairwell with her driver and two other wounded people before they ran into a bloody supermarket to escape through a loading dock.

Combined Kenyan military and police forces had the mall surrounded on Sunday. An Associated Press photographer saw Kenyan soldiers carry into the mall a rocket-propelled grenade, an extremely heavy weapon for an indoor hostage situation. Kenyan security forces are controlling the security cameras inside the building, Interior Cabinet Secretary Joseph Lenku said.

"The priority is to save as many lives as possible," Lenku said, reassuring the families of the hostages in the upscale Westgate mall. Kenyan forces have already rescued about 1,000 people.

Trucks brought in a fresh contingent of soldiers from the Kenya Defense Forces early Sunday.

The mall, which is in the Westlands neighborhood of Nairobi, is frequented by foreigners and wealthy Kenyans.

Security forces had pushed curious crowds far back from the building as the standoff ensued. Hundreds of residents gathered on a high ridge above the mall to watch for any activity.

Kenya has approached Israel for help on the standoff, and Israel sent an advising team.

U.S. law enforcement, military and civilian personnel in Nairobi were providing advance and assistance as requested by Kenya, spokeswoman Marie Harf said.

Kenyatta's nephew and the nephew's fiancee are also among the dead.

Kenya's presidential office said that one of the attackers was arrested on Saturday and died after suffering from bullet wounds.

British Foreign Secretary William Hague said late Saturday that his government had sent a rapid deployment team to Kenya to help. Britons had undoubtedly been caught up in the "callous and cowardly and brutal" assault at the Westgate mall, said Hague.

The United Nations Security Council condemned the attacks and "expressed their solidarity with the people and Government of Kenya" in a statement.