The Islamic Center of Lexington in Kentucky received an email threat on Friday telling everyone in the mosque would be killed.
"I'm going to kill everyone in there you Muslim (expletive). Salabis is coming for you," the email read, according to the Council on American-Islamic Relations (CAIR), the largest Muslim civil rights and advocacy organisation in the U.S., according to the Lexington Herald Leader.
Because of the threat, the centre contacted the police and federal authorities.
"We are aware of the incident and are looking into it to determine the proper course of action," said David Habich, chief counsel for the FBI in Louisville.
CAIR has requested law enforcement agencies to beef up protection for the mosque.
Imam Mahmoud Shalash said he does not know the meaning of "Salabis" but said the person sent the threat through an iPhone.
"Somebody has to be sick to reveal himself," said Shalash.
Lt. Joe Anderson of the Lexington Police Department said they are investigating the threat and patrols will be stepped up in the area around the mosque.
"This clear terroristic death threat targeting a house of worship must be treated seriously by local, state and federal law enforcement authorities and the perpetrator should be apprehended and brought to justice," said CAIR National Communications Director Ibrahim Hooper. "Our nation's leaders must send a clear message challenging the rising Islamophobia that leads to such threats."
He said last Friday, CAIR revealed an FBI alert to New York police about an Arizona anti-Islam extremist who is reportedly traveling to that state to "confront" Muslims.
Anti-Islam advocate Jon Ritzheimer, a former Marine, posted a video on Nov. 18 spewing expletives, showing a gun and making anti-Obama statements, reported the New York Daily News.
He once organised an armed protest at a Phoenix mosque and threatened to "arrest" all senators who voted to approve President Obama's nuclear deal with Iran.
Ritzheimer has been called the American Taliban by The Islamic Post.
The CAIR recently released a list of incidents of backlash and discrimination against Muslims in the U.S.
