Language and Police Work

 

According to the newspaper -

THE man who threw a hand grenade at the Carol's Billiard Hall in Block 4 of the Hilltop market last June 29 could speak Ilocano fluently, the police said. More...

Wow! That's some detective work! Considering that 90% of people in Baguio speak Ilocano and a few thousands speak it FLUENTLY. That really narrows down the search for the suspect.

But it still bothers me - How did they determine his fluency? And forgive my ignorance, but how does that help identify the suspect?

What would Sherlock Holmes do?