Dar Al-Hijrah Islamic Center in Falls Church, Virginia received an envelope about 5:45 p.m. Tuesday with bias language on it, Fairfax County police said.
"Kill all Muslims," a photo published by the Council on American-Islamic Relations (CAIR) shows on the envelope. Above the words is a drawing of a pig defecating on a mosque.
Anti-Muslim bigots often use pigs or pork products to offend Muslims, many of who do not eat pork for religious reasons.
The postmark on the envelope appeared to be from another country, police said. Detectives are investigating.
CAIR called on law enforcement officials to figure out who sent the note.
“We urge local, state and federal law enforcement authorities to treat this and other cases of threats and violence targeting American mosque seriously and to bring the perpetrators to justice,” CAIR spokesman Ibrahim Hooper said in a statement.
On Monday, two Jewish schools in Maryland and a third Jewish school in Virginia received bomb threats by phone, and a mosque in Maryland received a threatening letter. An Islamic center in Maryland received a similar letter on Tuesday.