Bullets were sent to five politicians in an escalation of the loyalist street protests in Northern Ireland.
Sinn Fein and cross-community Alliance Party representatives were targeted in packages posted to the legislative assembly at Stormont.
A string of attacks against politicians has followed the decision to limit the number of days the Union flag is flown from Belfast's City Hall.
The move sparked loyalist violence and road blocks, which have continued for most of this month, blighting the city's trading in the run-up to Christmas.
Gerry Kelly, Sinn Fein MLA for North Belfast, said: "This is obviously a step up in the attempt to intimidate elected representatives."
Sinn Fein said letters containing bullets had been addressed to assembly members Mr Kelly and Alex Maskey at Stormont.
The Alliance Party leader David Ford, East Belfast MP Naomi Long and councillor Gerardine Mulvenna were also targeted.
Mr Kelly said: "This will not deflect Sinn Fein from our work of seeking parity of esteem when it comes to reflecting people's Irishness and Britishness."
A spokeswoman for the Police Service of Northern Ireland said: "Police are dealing with a number of suspicious packages which have been located in the postroom at Stormont."
Unionist political leaders have unveiled proposals they hope will persuade loyalists to end their protests over the flag.
As violence spread throughout the country, a police officer was shot at in the Springhill area of Bangor yesterday.
Police have confirmed no-one was injured in the incident.
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