The letters were sent to churches and homes. Thousands of Christians live in the province, which is largely controlled by the Taliban and tribal authorities after the Pakistan government removed its troops from the area last year.
The Taliban religious police have been cracking down in the area ever since. Girls' schools have been burnt to the ground, video shops have been blown up, and there have been threats to local barbers whom they say insult Islam by shaving beards.
The police are reportedly investigating the source of the letters, and a local police official told the Associated Press that security has been stepped up at local churches.
The law-and-order situation in the Northwest Frontier Province has been steadily deteriorating. Within the last couple of weeks, there was a failed assassination attempt on the country's interior minister, a suicide bombing in which almost 40 people were killed. Earlier this week, a former minister and the leader of a secular party were killed.
Meanwhile, the government of Pakistan has been focusing its attention on a judicial crisis. Today, a security force of 15,000 is being dispatched in order to prevent further violence between supporters and opponents of Pakistan President Pervez Musharraf.