It was supposed to be a day where he focused on the exam paper in front of him, but Ustaz Mustazah Bahari found it hard to concentrate on Sept 12, 2001.

Some time after he started on his exam in a hall at the National University of Malaysia that morning, the headline of a newspaper that one of the invigilators was reading caught his eye.

On the front page were details about the horrific attacks that happened the day before. The then 24-year-old and budding ustaz, or Islamic religious teacher, could not believe his eyes, and he could no longer focus on the paper.

"This was a time before we all had Facebook or even the Internet we have today. That morning was the first time I heard of the attacks," says Ustaz Mustazah, who graduated with a master's in Quranic Sciences and Prophetic Traditions in 2005.

"I was shocked that something like that could happen - it did not feel real at all. My concentration was disturbed and I couldn't focus on the exam."

Immediately after the exam ended, Ustaz Mustazah went to buy a copy of the newspaper to find out what had happened. He also watched clips of the attacks on television.

As he caught up on the news, the Singaporean thought the problem of radical Islamism was something that was far away and would not affect the Republic or the region.

"But of course we know now that this is so far from the truth. The implications of that attack would be felt here too, and are still being felt to this day.

"The threat of radicalisation and extremists who want to use Islam for their devious purposes is always there," says Ustaz Mustazah, now 44 and married. The couple have a seven-year-old daughter.

He is a member of the Religious Rehabilitation Group, a voluntary unit made up of Islamic scholars and teachers in Singapore that works to rehabilitate radicalised people and their families through counselling.

The events on that day 20 years ago have left their mark on the Muslim community here, and Ustaz Mustazah says the threat of radicalisation is something that he warns against in almost all his religious lectures.

He stressed that it is something the community needs to be constantly aware of and take seriously.

"It is not a matter to be put aside. With every attack that we read about or see online, it's a reminder that extremist groups are out there and pose a great danger," he says.

"This is an issue that will keep on continuing."