Washington: Pakistan-based al-Qaeda engineers are working on a laser warning system and shoulder-fired missiles to counter American drones, The Washington Post has reported, citing classified documents it received from whistleblower Edward Snowden. Al-Qaeda affiliates around the world were also sharing lessons on basic defensive measures. The Post said that in January 2011, US spy agencies detected an unusual electronic signal emanating from near Miran Shah, in North Waziristan. The Defence Intelligence Agency called the signal “the first observed test of a new terrorist GPS jamming capability”, adding it did not pose a threat to military GPS frequencies or encrypted communications links.