We must never underestimate the influence and reach dark religious groups can have via the Internet ("Counter ISIS ideology on social media: Halimah"; Nov 22 and "Multi-pronged effort needed to take down ISIS" by Mr Pavithran Vidyadharan; Dec 4).

As proven elsewhere, educated and seemingly ordinary citizens can be transformed into highly irrational and dangerous people who act ruthlessly and senselessly ("California shooters radicalised for some time: FBI"; Wednesday).

The authorities must work closely with religious groups here to continually identify and block sites which use religion to incite hate and violence.

Religious groups and schools must also warn students about the dangers of visiting such websites.

Indoctrination, disguised as serious sermons, can influence our people and may have grave consequences. It is vital for the authorities to be alert, as we cannot afford to let our guard down.

Kang Choon Tian