Officers are thought to have been concerned that the Islamophobic "malicious communication" was linked to letters sent to Muslims in London, Yorkshire and the Midlands in recent days calling for April 3 to be Punish a Muslim Day.
Several ambulances from the Incident Response Unit - including hazardous substance specialists - were on the scene, and the police are now investigating the substance. Meanwhile, Britain's counter-terrorism officers continue to investigate after similar "Punish a Muslim Day" letters were sent to a string of locations across the country.
The London police announced on Monday that two policemen were "hospitalized" after discovering a suspicious package at the Norman Shaw Buildings, a pair of buildings in Westminster used as Parliamentary offices, reports AFP on Tuesday.
The Westminster offices of Rupa Huq, Mohammad Yasin, Rushanara Ali and Afzal Khan were all cordoned off while police investigated. The sickening point-scoring system ranges from 10 points if you "verbally abuse a Muslim" to 2,500 points if you "nuke Mecca". It offers 500 points for butchering a Muslim using a gun, knife or vehicle, and 1,000 points for bombing or burning a mosque.
A parliamentary spokesman said: "The police are investigating the content of the letters to identify any potential link".
Counter-terror officers are now investigating reports of similar letters in Yorkshire, with social media users also having reported receiving the letters in Birmingham and London.
"Are you a "sheep" like the vast majority of the population?"
"Sheep follow orders and are easily led, they are allowing the white majority nations of Europe and North America to become overrun by those who like nothing more than to do us harm and turn our democracies into Sharia-led police states. Do not be a sheep!"
Khan's package was unopened by his staff but the package to Huq was opened and a member of staff was taken to hospital as a precaution after opening the letter, which the MP said contained a "sticky substance" that the police had described as "low-level noxious". It seems, though, that the writers targeted predominantly Muslim communities.
'And we must pull together and ensure that this does not happen - not only here but also throughout the country'.
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