(NBC News) When U.S. Navy SEALs stormed a compound in Abbottabad to kill Osama bin Laden on May 2, 2011, two young female polio workers – Sharafat Bibi and Sunbal Bibi – had no idea it would one day be a justification for their murder.

Sharafat and Sunbal were killed by unidentified armed men on May 28 in a village in Peshawar, the capital of Khyber Pakhtunkhwa province, where they were administering anti-polio vaccines to the children.

They are among 20 people, three of them security personnel, who have been gunned down by suspected militants during the last two years for vaccinating children against polio.

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