GWEN IFILL: Orlando, a city that evokes theme parks and good times, now also a blood-stained entry in American history, 49 people, plus the killer, slain at a gay nightclub. Sunday's massacre led today to a search for motive and for missed signals.

William Brangham begins our coverage in Orlando.

WILLIAM BRANGHAM: There was relative calm outside the Pulse nightclub this morning, a far remove from the chaos of 24 hours earlier. Amateur video captured the terror, gunshots shattering the party atmosphere inside the club.

The man firing the shots was 29-year-old Omar Mateen, ultimately killed by a SWAT team inside. Police said today they have no regrets about storming the club.

JOHN MINA, Chief, Orlando Police Department: Based on information we received from the suspect, and from the hostages, and people inside, we believed further loss of life was imminent. I made the decision to commence the rescue operation and do the explosive breach.

WILLIAM BRANGHAM: During the attack, Mateen called 911 dispatchers and pledged loyalty to the Islamic State.

Today, ISIS radio released an audio statement calling him a soldier of the caliphate. And officials in Saudi Arabia confirmed he'd visited their country twice for pilgrimages.

But, in Washington, FBI Director James Comey said there's every reason to think Mateen acted on his own.

JAMES COMEY, Director, FBI: There are strong indications of radicalization by this killer and of potential inspiration by foreign terrorist organizations. So far, we see no indication that this was a plot directed from outside the United States, and we see no indication that he was part of any kind of network.

WILLIAM BRANGHAM: The FBI had investigated Mateen on suspicions of terrorist sympathies, but the results were inconclusive, and Comey defended his agents' work.

JAMES COMEY: Our investigation involved introducing confidential sources to him, recording conversations with him, following him, reviewing transactional records from his communications, and searching all government holdings for any possible connections, any possible derogatory information.

WILLIAM BRANGHAM: As investigators now searched the gunman's home in Fort Pierce, Florida, sharply differing images of him have emerged. His Afghan-born father apologized again today to the victims.

And in a Facebook video, speaking in Dari, he reflected on his son and the crime he committed.

SEDDIQUE MIR MATEEN, Father of Omar Mateen: My son Omar a good son and educated person. I don't know what caused him to do it. I don't know what happened. And I didn't know he had hatred. The issue of homosexuality and punishment belongs to God. It doesn't belong to a servant of Allah

WILLIAM BRANGHAM: But Mateen's ex-wife painted a starkly different picture, saying he was abusive, mentally unstable and full of hate.

At the White House, President Obama spoke of his own concerns about such individuals.

PRESIDENT BARACK OBAMA: One of the biggest challenges we are going to have is this kind of propaganda and perversions of Islam that you see generated on the Internet, and the capacity for that to seep into the minds of troubled individuals.

WILLIAM BRANGHAM: Meanwhile, police said today they have identified nearly all of the victims, among them, accountants, baristas, hairdressers, people, mostly young, who'd been looking to unwind with a night of music and dancing.

With the city's wounds still fresh, Orlando's Mayor Buddy Dyer thanked the city for its response.

MAYOR BUDDY DYER, Orlando: We will not be defined by the act of a cowardly hater. We will be defined by how we respond, how we treat each other. And this community has already stepped up to do that.

WILLIAM BRANGHAM: As the country awaits the latest developments in this investigation, a small army of volunteers here in Orlando have come to this community center to do what they can to help out.

They loaded food and water today at Orlando's biggest LGBT community center, just minutes from the site of the rampage. Many said they came to lend a hand, but also just to be with others in mourning.

COREY LYONS, President, Impulse Group Orlando: It's tragic. It's tragic across the board. We saw the act of what one person can do, but in the same day within 12 hours, we saw the acts of what many can do together to really fight this incident and really try to come together.

CHRISTIE CRUZ, Volunteer: We're all people, and it just hurts cause it's black, white, gay, no matter what. It doesn't matter,

WILLIAM BRANGHAM: It's an attack on everybody.

CHRISTIE CRUZ: It's just — yes, we're all people, we're all human, and we — it's just — it's hard to deal with. That's the thing about the gay community, is, we all know somebody who knows somebody who knows somebody, because this is a close group, and we're — it hurts us all.

WILLIAM BRANGHAM: The shooting happened as Latin Night at the Pulse nightclub was winding down, and a majority of the victims identified so far are Hispanic.

Christian Castelan is Mexican-American and gay.

CHRISTIAN CASTELAN, Volunteer: I went out to dinner last night with my significant other and other friends, and just being closed to him, that is even scary, after what happened, because I was constantly just looking around. It just went from completely comfortable to just completely fearful in a matter of hours. And it just continues to become much more difficult than what it was yesterday.

WILLIAM BRANGHAM: A vigil is being held later tonight for the victims of the shooting. It's about less than a mile away from the nightclub where they lost their lives — Gwen.

GWEN IFILL: William, I just heard that young man said to say to you that they went from being comfortable to terrified so quickly. Is it fair to say that the community is still in a state of shock tonight?

WILLIAM BRANGHAM: Absolutely. I mean, the sense here is that this, like so many other communities that have experienced this type of a tragedy, the sense of safety that they had has been snatched away from them.

And that's something we heard not just from the gay community, but largely in that community here, all over Orlando.

GWEN IFILL: So, William, is there a special impact in the gay — if this was Latin night at a gay nightclub, a special impact in the gay and lesbian community?

WILLIAM BRANGHAM: It was, absolutely.

I mean, the Latino community, as we know, it's largely a conservative community. And being gay in the Latino community is not as easy as it might be, say, in the white community. And a lot of people mentioned to me that there might be the possibility that people who were shot, people who might have been killed or wounded in this event might not have been out even to their own families.

And so now they are forced to have this conversation that maybe they weren't ready to be having, that, oh, I was at this club when this happened.

It just raised one more level of a very, very uncomfortable conversation for them at a time of this awful tragedy going on here.

GWEN IFILL: And is there evidence now of heightened security around, in and around Orlando, but especially in that neighborhood?

WILLIAM BRANGHAM: Absolutely. We spent a good deal of time today and yesterday at that community center.

And now even there every time they don't let people stand outside, for fear of drive-by shootings. They are checking bags everywhere they go. How much of that lasts after this event dies down and people start to go on with their lives, we don't know. But there is a very strong sense of security here everywhere.

GWEN IFILL: William, thank so much. We will hear from you again tomorrow night.

WILLIAM BRANGHAM: All right, good night, Gwen.