DELHI, INDIA – A suicide bomber entered a crowded mosque in northwest Pakistan and detonated an explosive that killed more than 50 people during Friday prayers.

It's unclear who was behind the attack, which took place in tribal region West of Peshawar known as the Khyber Agency. Security forces tasked with protecting NATO supply lines were among those praying and subsequently killed at the mosque, according to local reports.

By attacking a packed Sunni mosque at Friday prayers, the perpetrators showed a brazen disregard for popular sentiment in the region, analysts say.

"I think people in the frontier will be scared to send their children and men to the mosque. And I think the moderate sort of religious people in Pakistan will be calling for some action against these people," says security analyst retired Brig. Gen. Mahmood Shah.

Earlier this month, militants bombed a Sufi shrine in Peshawar. The attack damaged the tomb of Abdur Rahman Baba, a Sufi saint and one of the most famous poets in the Pashto language. Many local Pashtuns are fond of reciting his verses. Although that attack struck at a popular ethnic figure, it was largely seen as a sectarian strike against the Sufi faith.

Today's attack marks the first bombing of a Sunni mosque in the region, says Mushtaq Yusufzai, a Peshawar-based reporter with The News, a newspaper based in Pakistan. Although the attack will further anger local people, Mr. Yusufzai doesn't see that turning into an effective backlash against militant groups.

"The people cannot react, they are helpless. They cannot fight against these unseen fighters," he says. In the past, local tribal leaders have organized lashkars, or armed posses, to go after militant groups. But militants retaliated by killing tribal elders and burning their homes, breaking much of the local resistance.

Yusufzai said he talked to some locals who claim to have seen drones recently in area – the rumors, valid or not, will no doubt add to growing resentment of US airstrikes in the region. He also said the attacks came as a surprise because local Taliban were believed to have agreed to focus their efforts on the fight in Afghanistan.