Somalia-based al-Shabaab says it is responsible for the Saturday attack at the Westgate Mall in Nairobi that left at least 62 people dead and 200 wounded.
U.S. intelligence officials and the FBI said Monday they are "aggressively" investigating if any Americans were among the attackers, Fox News reported.
U.S. officials say up to 50 U.S. citizens have traveled to al Shabaab training bases over the past six years, ABC News reported Sunday.
Three men who purportedly were from Minneapolis appeared in an al-Shabaab video last month urging others to join them in Somalia.
"This is the real Disneyland, you need to come here and join us and take pleasure in this fun," one of the men said.
The video said the men have since died as martyrs.
Another American, Omar Hammami of Daphne, Ala., rose to a leadership position in al-Shabaab before rivals reportedly killed him.
U.S. officials worry that Americans recruited by al-Shabaab may return home and carry out terrorist attacks.
"What we see today in Kenya could be easily copied here in the United States," said Ali Soufan, a former FBI agent and expert on terrorism.
A recent analysis by the U.S. government has not found any heightened interest by the terror group in striking at the U.S. homeland, said a senior law enforcement official. While al-Shabaab desires to attack the United States, it's not a priority for them, he said.