A civil rights group has asked the FBI to investigate a threatening letter sent to a Phoenix mosque that was once a place of worship for suspects in a shooting three weeks ago at a Texas cartoon contest featuring cartoons of the Prophet Muhammad.

The Council on American-Islamic Relations says one of its lawyers spoke with an FBI agent about the letter that was mailed over the weekend to the Islamic Community Center of Phoenix, where Texas shooting suspects Elton Simpson and Nadir Soofi used to worship.

An identical letter also was sent to the Islamic Community Center of Tempe, said Kristy Sabbah, operations coordinator for the advocacy group's Arizona chapter.

Simpson and Soofi opened fire on May 3 in a Dallas suburb on an unarmed security officer stationed outside the contest, authorities said. The security guard was wounded in the leg before the gunmen were killed at the scene.

Simpson regularly attended the Phoenix mosque for about 10 years, but he quit a few months before the shooting. Soofi infrequently attended the mosque and stopped altogether about a year ago.

The letter opens with a reference to Simpson's and Soofi's connection to the Phoenix mosque, threatens the lives of the mosque's president and his family, and makes a disparaging comment about the Prophet Muhammad.

The FBI office in Phoenix didn't immediately return a message Tuesday morning.

Usama Shami, president of Islamic Community Center of Phoenix, said the Phoenix mosque gets threats from time to time but that the letter addressed to him contains threats that were more violent in nature.

"We are not educating people to do violent acts, so why are we being held responsible in the minds of some people?" Shami said.