There was no clear evidence that any Americans were involved in Saturday’s armed rampage and hostage siege at Kenya's Westgate complex, a U.S. intelligence official told NBC News.
Al Shabaab, the Somalia-based militant group that claimed responsibility for the massacre of at least 72 people at the mall in the country's capital, says Americans were involved in the attack. However, a number of the names provided by the al Qaeda-linked group are not turning up as connected to any real person, the official said.
While dozens of U.S. investigators arrived in Kenya over the past few days, U.S. officials have publicly steered clear of saying whether or not there has been a link to the United States.
"We don't have any verifiable information that would indicate one way or the other way that Americans were involved," U.S. Attorney General Eric Holder said Thursday.
American officials remained very skeptical of reports that Americans were involved in the attack, according to Michael Leiter, NBC News' counter-terrorism analyst.
"The U.S. intelligence community has not found any evidence or intelligence confirming the presence of U.S. citizens in this attack. That doesn't mean that there aren't but as of today there's no clear evidence that they were," he said.
Nevertheless, the mall assault showed al Shabaab has a "concerning ability" to strike outside its base of Somalia and stage a sophisticated, multi-layered attack, House Intelligence Committee Chairman Mike Rogers said on Thursday.
The Michigan Republican, who has been briefed by U.S. intelligence officials, said the "bold attack" bore "striking similarities" with al Qaeda's deadly multi-day siege of an Algerian natural gas facility in January, according to The Associated Press.
He said the tactics used -- including hostage-taking, multiple gunmen and small explosives and setting fires to cause panic -- indicated the attackers planned the operation well in advance and had studied their target for the best time to strike.
"Groups like al Shabaab are really the new face of al Qaeda," Leiter said. "And the attack that we saw in Nairobi represents the worst face. But what we have to make sure is that those threats remain regionally focused and not towards the West and the United States."
Meanwhile, blasts were heard overnight at the Westgate mall in Nairobi overnight.
Security sources said these were controlled explosions as security swept the complex ahead of international forensic teams, which were planning to go into the complex on Friday.
On Thursday, international crime fighting organization Interpol has issued a “Red Notice,” or internationally wanted persons alert, for the British mother of three dubbed the “White Widow” at the request of authorities in Kenya.
Samantha Lewthwaite, known as "Sherafiya," is thought to be a key member of al Shabaab, the militant group that has claimed responsibility for the deadly Nairobi mall siege.
She is wanted by Kenya on charges of being in possession of explosives and conspiracy to commit a felony dating back to December 2011, according to Interpol.
Circulated to all 190 Interpol member countries, the Red Notice is one of the most powerful tools in tracking international fugitives.
NBC News' Alastair Jamieson, Mary Murray and Henry Austin contributed to this report.