Hours after the December shootings in San Bernardino, Calif., Mark Wallace asked his employees at the nonprofit Counter Extremism Project to comb social media for profiles of the alleged attackers.

They failed. A team at Facebook Inc. had already removed a profile for Tashfeen Malik, after seeing her name in news reports.

The incident highlights how Facebook, under pressure from government officials, is more aggressively policing material it views as supporting terrorism. The world's largest social network is quicker to remove users who back terror groups and investigates posts by their friends. It has assembled a team focused on terrorist content and is helping promote "counter speech," or posts that aim to discredit militant groups like Islamic State.

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