Significance of Orlando gunman calling 911 during standoff

The FBI said investigators are following up on about 100 leads in the Orlando attack that killed 50 people, including the gunman, Omar Mateen. It is the deadliest mass shooting in U.S. history.

The 29-year-old shooter opened fire around 2 a.m. ET at Pulse, a nightclub that's described itself as Orlando's hottest gay bar. He called 911 during his nearly three-hour standoff with the police, holding several hostages and pledging allegiance to the Islamic State of Iraq and Syria (ISIS).

"I think that's very significant because ISIS tells its followers that they must pledge bayat, they must pledge allegiance to ISIS before they die. We saw that in San Bernardino, and now we've seen it here," former CIA deputy director and CBS News senior security contributor Michael Morell said Monday on "CBS This Morning." "So I think it's very important because it shows that he was in touch with that ISIS messaging."

While ISIS has claimed credit for the attack, it's not clear whether the attack was directed by ISIS or was a lone-wolf incident, Morell said. Investigators have yet to find clear links between Mateen and ISIS' Syria-based terrorist leadership.

"There is no evidence that he was directed, but we still don't know that," Morell said.

Morell also said there was "no doubt" Mateen prepared for this attack.

"It appears that he surveilled the location ahead of time. He must have thought through that 911 phone call. So this was premeditated in every meaning of the word," Morell said.

Mateen, who was born in New York to Afghan parents, was on the FBI's radar twice, once in 2013 for inflammatory comments he made to co-workers about terrorist ties, and another in 2014 regarding possible connections to the first known American suicide bomber in Syria, Moner Abu-Salha. However, both cases were closed.

"The FBI literally investigates things like that thousands and thousands and thousands of times. They simply found nothing to follow up on to open up a bigger investigation," Morell said.

A law enforcement source said Mateen had recently legally purchased the two weapons used in the attack: a Glock 17 handgun purchased June 5, and a Sig Sauer MCX assault rifle purchased on June 4. Investigators also found a .38-caliber weapon in his vehicle.