Odd way of saying “thank you.”

MARAWI CITY — Twenty years after obtaining Christian education, Muslim brothers Omarkhayam and Abdullah Maute returned to this Central Mindanao city with the black flags of the Islamic State (IS) group and set fire to their alma mater.

Hundreds of gunmen, many of them young locals recruited by the Maute brothers, destroyed Dansalan College in a rampage through Marawi City last month as they launched a brutal offensive to stamp their credentials as the IS leaders in the Philippines.

Their bold attack turned the Maute siblings, who are in their mid-30s, into the most infamous high school alumni of the Protestant Church-run institution, which had been a symbol of religious tolerance in the mostly Muslim city of 200,000 people.

The Maute brothers have since remained holed up in Marawi, using their local knowledge of tunnels and bombproof basements to withstand a military offensive that has left entire neighborhoods in ruins and claimed more than 300 lives.