ONTARIO (AP) — Pig's legs were thrown on the proposed site of a Southern California mosque, and a Muslim civil rights group is asking federal officials to investigate.

Worshippers said two women threw the three legs onto the driveway of the proposed Al-Nur Islamic Center in Ontario shortly before 10 p.m. Tuesday and sped away in a white pickup, said Munira Syeda, spokeswoman for the Greater Los Angeles Area chapter of the Council on American-Islamic Relations.

Pork is forbidden to practicing Muslims and pigs are considered unclean animals.

"This is Ramadan. It's the holiest time of the year for most Muslims, so it's especially offensive," Syeda said Friday.

The site is a house where Muslims gather to worship until a mosque is built there, Syeda said.

A security guard saw the incident and a vandalism report was filed with the San Bernardino County Sheriff's Department.

No arrests have been made, sheriff's spokeswoman Cindy Bachman said.

In a letter sent to the U.S. Department of Justice on Thursday, the council urged the government to open a hate-crime investigation.

"Only firm action by local, state and federal law enforcement authorities will send the message that bias-motivated attacks on Muslims, or Americans of any faith, will not be tolerated," CAIR-LA Executive Director Hussam Ayloush said in a statement.

No investigation has been opened, but it could be in the future if officials believe it is warranted, said Thom Mrozek, spokesman for the U.S. attorney's office in Los Angeles.

Plans for the proposed mosque are still in the approval process and some residents have expressed opposition, Syeda said.

Some worry that the mosque will create traffic and other problems, but some simply do not want a Muslim house of worship in the city, she said.

"Unfortunately, there's a wave of anti-Islamic sentiment around the country," Syeda said. "It's a time for Americans to remember our values of pluralism and diversity."

The vandalism came only days after four teenagers were arrested for allegedly throwing lemons and oranges at a mosque in the San Francisco Bay area of Hayward. It occurred last Saturday during prayers at the American Muslim Association.

Four Hayward boys, ages 13 to 16, were arrested on suspicion of vandalism that interfered with the civil rights of the worshippers.

It was one of several recent vandalism attacks on the mosque, authorities said.

CAIR's national headquarters issued a community safety advisory for U.S. mosques this week following "other incidents targeting mosques" in Missouri and Rhode Island and after the deadly shooting at a Sikh temple in Wisconsin, the group said.