Every morning since 9/11 (which my brother miraculously survived), I have woken in fear of another nightmare on American soil.

After Sandy Hook, I became afraid that my sons, Isaac, 4, and Albert, 13, would be shot, like the 26 trusting grade school children. Those lives, those eager learners, were taken away, just as they began their education.

On June 12, my fear reached the point where I can no longer listen to the morning news on television.

On that day, my wife woke me. We thought of celebrating Father’s Day early at the Hotel Marriott, but I have never been a fan of hotels or clubs. I don’t go to places like Pulse, the gay nightclub in Orlando, Fla. Instead, I took Isaac to Lehman College.

Isaac is often called the 4-year-old Einstein. He expected to solve math problems at the college, prior to his interview on differential calculus with Dr. Ricardo Fernandez, the President of Lehman College.

At the college, the security officers, familiar to us from our many visits, stopped us and asked where we wanted to go. I was surprised, because the same guards had helped Isaac many times previously. So, I thought, they wanted to know which classroom needed opening.

Instead, they interrogated us. I reminded the officers that a banner with my picture was hanging on the campus. They insisted the questions were mandatory.

We complied. I noticed other officers, inside the security booth, staring at us. When the guards started patting-down Isaac, I realized something was wrong. On a TV monitor in the security booth, I saw a man, who I now know was Omar Mateen, armed with a semi-automatic assault rifle, indiscriminately shooting everyone in the packed Pulse nightclub.

I had watched a similar incident on television on December 16, when seven terrorists killed more than 130 children at a Pakistan army school. My heart dropped and my whole body quivered. I hoped that Mr. Mateen would not be Muslim, as I am. A few seconds later, I heard Mr. Mateen saying “Allahu akbar,” and I struggled to stop the tears.

The security officer searching little Isaac looked at me and asked, “What does Allahu akbar mean?” I wanted to say (but did not), “Allahu akbar used to mean, ‘God is great.’ Today it means, ‘I am about to use hijacked planes to fly into the Twin Towers.’ It means, ‘I am about to kill hundreds of schoolchildren.’ It means, ‘I’m about to take away the lives of 50 innocent people in the Pulse nightclub, and wound 53 more’.”

In my head, I said to Mr. Mateen:

You killed 50 people to make Allah happy? You made the reputation of Muslims worse than Hitler did.

You killed the dancers at the club to make the Prophet happy? You made people think your fellow Muslims are terrorists, but it’s you.

You killed innocent people to force the U.S. to respect Islam? You just jeopardized 1.8 billion Muslims.

You wanted to destroy diversity? You made me appreciate American diversity even more. You made me love Pulse. You made me love gay people.

When I saw Mr. Mateen shooting innocent people, I whispered, “Omar Mateen, you are not Muslim. You are not a human being. You are an animal. You are a disgusting bastard.”

I am a Muslim, and I love the LGBT community. I love all human beings, of all religions, ethnicity, and cultures. I am not only a Muslim; I am also a Hindu, Buddhist, Jew, and Christian.

That day, we solved no math problems at the college. On our way home, I wondered what Mr. Mateen hoped to teach the U.S. by killing innocent people. He may have tried to trick the American people into believing that he represented more than a billion Muslims. It seems he fooled Donald Trump. But, President Obama was right when he said, “Groups like [Islamic State] … (falsely) claim that they are the true leaders of over a billion of Muslims around the world who reject their crazy notions.”

After the horrific shooting at Pulse, the question is obvious: How can we prevent a domestic terrorist act in the future? How could Mr. Mateen, who the F.B.I. interviewed twice, buy two assault rifles? There is something deeply wrong with our law that allowed this crime to be committed. Incidents like the Pulse massacre occur because lawmakers hesitate to restrict gun ownership. Buying guns is as easy as buying cough medicine. Congress should understand Hillary Clinton, who said, “If the F.B.I. is watching you for a suspected terrorist link, you shouldn’t be able to just go buy a gun with no questions asked.”

When we got home, around 10 a.m., my elder son Albert opened the door for me, and gave me an early Father’s day gift. The horror flooded back to me. I wondered who would open the doors for the fathers who lost their children at Pulse. Who will give them a Father’s Day gift? I will celebrate by holding Isaac and Albert’s hands in my backyard. The fathers of the victims at Pulse will go to the graveyard.

Rashidul Bari teaches mathematics at CUNY-Baruch College and physics at New Visions Charter High School for Advanced Math & Science. His anti-Islamic State website is: www.bari-science-lab.com