Two packages were sent to the same building in Parliament on Monday afternoon.
An urgent question was raised in the House of Commons by Labour MP Yasmin Qureshi after reports that letters were sent out in a number of United Kingdom cities, calling on people to verbally and physically attack Muslims.
The leaflets, reported in West Yorkshire, Midlands, London, Cardiff, Leicester, Sheffield and Birmingham, carried an image of a dagger with the letters MS, indicating links to so-called Muslim Slayer which targeted mosques in London and the United States past year with similar threats, the Mirror reported.
Her staff said they were "thrown into a panic" by security after the incident, but the Home Secretary was out of the country.
"The issue of anti Muslim and far-right extremism is a focus for the government", she said, pointing to the government's anti-radicalisation programme, Prevent, which she said did not focus just on Islamic radicalisation but other forms of extremism, including of the far right.
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"It is to put the frighteners on us but we won't be cowed".
MPs have condemned letters encouraging recipients to take part in "Punish a Muslim Day" as "abhorrent".
Urging people to join their "day of action" next month the letter reads: "They have hurt you, they have made your loved ones suffer".
A third Muslim member of parliament from the party, Mohammad Yasin, said he also received a package on Monday.
In a statement West Midland Police said asked anyone who had received one to get in contact.
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Scotland Yard announced that an anti-terrorism investigation was under way to identify the ones behind these letters.
A Westminster parliamentary spokesperson confirmed four packages had been sent to the Norman Shaw north building and police were investigating the latest package.
The organization said that some of the letters appear to have been mailed out of Sheffield, the NYT reported.
Labour MP David Lammy tweeted: "I am absolutely appalled and horrified to hear reports that a number of my Labour colleagues are receiving "Punish a Muslim" letters, hate mail and suspicious packages".
A Parliamentary spokesman said: 'The Met Police are now investigating suspicious items of mail in Norman Shaw North.
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