GEOFF BENNETT: Welcome to the "News Hour."

Investigators are looking deeper into the motives of two suspected teen gunmen who opened fire at an Islamic center in San Diego yesterday.

Law enforcement officials say they were radicalized online.

More than 30 guns along with a crossbow were found at their residences, as was a manifesto with hateful and white supremacist views targeting a range of groups.

All of this comes as the community there grieves its losses.

The growing memorial in San Diego's Clairemont neighborhood reflects a community in mourning after a deadly attack on the city's largest mosque.

The attack on the Islamic Center of San Diego began yesterday just before noon local time, when two teenagers opened fire, killing three people.

Authorities said today the two gunmen met online and that law enforcement had found a manifesto targeting many groups, including Muslims.

MARK REMILY, FBI Agent in Charge: We also identified writings and various ideologies outlining religious and racial beliefs of how the world they envisioned should look.

These subjects did not discriminate on who they hated.

And let me be very clear.

To anyone that thinks that they can end the world through violence, they are sorely mistaken.

GEOFF BENNETT: Imam Taha Hassane paid tribute to the three victims, Mansour Kaziha, Nader Awad, and Amin Abdullah.

All, he said, were closely tied to the mosque.

IMAM TAHA HASSANE, Islamic Center of San Diego: My community is mourning.

My community is keeping our three heroes in their prayers.

My community is around the three families, trying to show support and sympathy.

GEOFF BENNETT: According to police, mosque security guard Amin Abdullah initiated a lockdown before he died in a shoot-out with the suspects.

SCOTT WAHL, San Diego, California, Police Chief: His actions, without a doubt delayed, distracted and ultimately deterred these two individuals from gaining access to the greater areas of the mosque, where as many as 140 kids were within 15 feet of these suspects.

GEOFF BENNETT: Law enforcement arrived within minutes, escorting students from the center's day school to safety; 9-year-old Odai Shanah was among the dozens of children forced to hide in classrooms as gunfire echoed through the complex.

ODAI SHANAH, Student, Bright Horizon Academy: I heard like a bunch of bad stuff like gunshots.

And, plus, I went out -- I went inside the closet with my whole class.

We heard like 12 or like 16 gunshots.

And then the SWAT team said, "Open up."

Then they opened the door.

GEOFF BENNETT: Police say the gunmen, aged 17 and 18, also fired at a landscaper working down the street, though he was not hit.

Authorities later found both teenagers dead inside their vehicle from apparent self-inflicted gunshot wounds.

San Diego police say they received a call two hours before the shooting from the mother of the 17-year-old gunman.

SCOTT WAHL: And she's beginning to provide that information to us, and it went from a runaway suicidal individual to homicidal, potentially homicidal two suspects.

GEOFF BENNETT: The attack came just days before one of Islam's holiest holidays.

Many in the community say they are now living with immense fear.

SARAH ELFEKY, Member, Islamic Center of San Diego: Immense shock, panic of, oh, my God, like, are my kids safe, are our kids safe?

Because these are our kids.

These are my sisters and brothers in humanity.

GEOFF BENNETT: Attacks on houses of worship remain relatively rare in the U.S., but incidents targeting religious communities have risen in recent years.

Last year, six people were killed in shootings at churches in Michigan and Minneapolis.

Temple Israel Synagogue in Detroit, which was targeted in an attack two months ago, said in a statement: "The images coming from San Diego are all too familiar to us."

The synagogue also called for $1 per month to protect houses of worship.

As the community mourns, the San Diego Sheriff's Department says it has increased security around houses of worship across the region.

This shooting at San Diego's Islamic Center has understandably sent shockwaves to the Muslim community here in the U.S.

For more, we turn now to Edward Ahmed Mitchell, deputy director of the Council on American-Islamic Relations.

Thank you for being with us.

EDWARD AHMED MITCHELL, Council on American-Islamic Relations: Thank you for having me.

GEOFF BENNETT: How has this shooting in the broader rise in anti-Muslim incidents across the country affected Muslim communities psychologically and emotional?

EDWARD AHMED MITCHELL: Anti-Muslim bigotry in the United States is completely out of control.

We have elected officials, members of Congress, governors who have in recent months said that American Muslims should be destroyed, that Islam has no place in America, that we should ban the practice of Islam, that mosques are military outposts.

Just last week, at a congressional hearing, that language was used.

And so, when you see that sort of anti-Muslim hate, it's no surprise that someone took it very seriously and engaged in a horrific act of violence.

So American Muslims, sadly, are accustomed to this.

This mosque had security there for a reason, because we know about the potential threat.

Now we feel, after this incident, it's time for this to end.

It's time for Islamophobia to no longer be the last publicly acceptable form of bigotry in our country.

And that starts from the top down.

GEOFF BENNETT: To your point about the level of security, we heard from the imam of the Islamic Center.

He said the center had done everything possible to prevent that kind of attack.

They applied for DHS grants.

They employed armed security.

They conducted drills.

They had cameras inside and out.

What challenges do mosques, houses of worship face when it comes to protecting themselves in this day and age?

EDWARD AHMED MITCHELL: Yes.

Well, look, a house of worship is not a military fortress.

It's a place where people come to worship, whether you're going to synagogue, a church, or a mosque.

And so it's very difficult to impose significant security restrictions without interfering with its ability to be welcoming to people of different faiths and, of course, the people there to worship.

Having said that, it's absolutely a must, especially for mosques and other targeted houses of worship, to have security cameras, to have an armed guard, especially for major events, and especially if there's a school there.

And we saw the benefit of this.

Amin Abdullah, the security guard there, saved countless lives by combating these attackers, by engaging in a firefight with them, and scaring them away, and losing his life in the process.

And so I can only imagine what would have happened if he hadn't been there at all.

But it just goes to show you that even a security guard cannot stop the violence.

That's why we have to stop the root of it, which is anti-Muslim hate, rampant anti-Muslim hate, tolerated by our government, spread by our government and certain media personalities.

Addressing that is the best way to stop this ongoing threat to mosques.

GEOFF BENNETT: You mentioned the security guard.

When tragedies like this happen, we all too often focus on the gunmen, the motives.

There's usually less attention on the victims.

What more should we know about Amin Abdullah, this father of eight?

EDWARD AHMED MITCHELL: Well, there were three members of the San Diego Muslim community murdered in this incident.

Amin Abdullah, obviously, as the security guard, has been widely recognized for saving lives, but also the other two gentlemen there.

One of them actually, we're told, ran into the building after Amin was killed and went into the building trying to help, and he himself was killed as well.

Another brother there was just killed as a bystander.

And so all three of these men were beloved, respected members of that community.

The Islamic Center of San Diego is one of the most prominent mosques in California and across the country.

And so this attack is horrific, and that's why the community is standing strongly with the families of the victims and doing everything they can to make sure that they are supported in this horrible time, and also to make sure that this doesn't happen to anyone else again in the future, God willing.

GEOFF BENNETT: In the roughly 30 seconds we have left, the Associated Press reports that the manifesto that was left behind had hateful rhetoric toward Jewish people, Muslims, Islam, the LGBTQ community, Black people, women, the political left and right.

At this point, what's the path forward here?

What's it going to take to really lower the temperature?

EDWARD AHMED MITCHELL: Yes, look, this starts from the top down.

As I said, if the president of the United States is engaging in open bigotry against various communities and members of Congress and governors in Texas and Florida, other states are doing that, it's no surprise you're going to see hateful rhetoric turning to hate crimes.

So we have to start with our political leaders.

They have got to stop fomenting hate against various Americans.

This is all our country.

Everyone has the right to be here and live in peace and worship in peace.

And so we need our political leaders to stop spreading hate, because it is endangering lives.

GEOFF BENNETT: Edward Ahmed Mitchell, thank you for your time this evening.

We appreciate it.

EDWARD AHMED MITCHELL: Thank you.