Though al-Shabab, meaning "The Youth," has carried out hundreds of deadly attacks against Somali government and African Union troops and a few attacks inside Kenya -- including suicide bombings perpetrated by several American recruits -- none were comparable to Westgate.
Only a 2010 bombing of civilians watching the World Cup in Kampala, Uganda, which killed one American, rivaled the level of operational planning and security the Westgate attack had to have required, experts said.
Before the Uganda attack, al-Shabaab moved weapons across the border and operated safehouses in both Somalia and Kenya, a retired FBI agent involved in that investigation said.
"This operation [at Westgate] required guns, grenades and people – and lots of planning," the U.S. official said.
Despite Kenyan officials' statements that American recruits may have been involved in al-Shabab's weekend assault on Westgate, U.S. officials have maintained they have nothing to back that up yet.
At a briefing with reporters Wednesday, Attorney General Eric Holder said, "We don't have any verification that any Americans have been involved in the attack as attackers, and so we are still working with our Kenyan colleagues to get granularity and fidelity as to what the truth is to that matter."
A U.S. military expert on al-Shabab said new consideration must be given to al-Shabab as a threat to the West – including the U.S. homeland potentially – because al-Shabab has clearly "crossed the line on external operations" with Westgate, given its brazenness, sophistication and success.
"It's a wakeup call to western intelligence services," the military source told a ABC News.
ABC News' Mike Levine contributed to this report.