(CNN) The author addressed the letter "to the children of Satan" and called Muslims "a vile and filthy people."

"There's a new sheriff in town," the letter said, "President Donald Trump."

An anonymous group calling itself "Americans for a Better Way" has sent a letter to at least five California mosques and another one in Georgia, according to the Council for Islamic Relations (CAIR), a Muslim advocacy group.

"He's going to do to you Muslims what Hitler did to the jews [sic]," the letter said. "You muslims [sic] would be wise to pack your bags and get out of Dodge."

Anti-Muslim crimes on the rise

The letters are the latest in a growing number of Islamophobic crimes during the past few years. From 2014 to 2015, anti-Muslim crimes in the US rose by two-thirds, according to the FBI's latest annual hate crime report.

Mark Potok, a senior fellow with the Southern Poverty Law Center, said such crimes have risen to the highest level since the 9/11 terror attacks.

Following the presidential election, the SPLC has documented more than 700 incidents of hateful harassment or intimidation against minorities. According to CAIR, at least 100 anti-Muslim incidents have taken place during that same period.

And after Islamic centers in Signal Hill, Pomona, Northridge, Granada Hills and San Jose received copies of the vitriolic letter, CAIR has called for "stepped-up police protection of local mosques."

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The Pomona Mosque said on its Facebook page that it received the letter Friday.

In an online statement , the mosque in San Jose confirmed it received a copy of the letter Thursday night.

"We informed the proper authorities and San Jose police have started a formal investigation," the statement said.

The San Jose Police Department, referring to the letter as a "hate motivated" incident, said in a statement it would conduct a follow-up investigation.

The letter: 'an infection' in the country

At a press conference Monday, law enforcement officials and Muslim leaders in Los Angeles gathered to address the letter's contents.

John Stedman, a commander with the Los Angeles County Sheriff's Department, encouraged the affected communities to report any letters they received either in person or online to authorities. No incident is too small, federal and local authorities echoed.

LAPD Deputy Chief Michael Downing, who specializes in counterterrorism, said the incidents are "caused by people who are fearful of communities that don't look or behave like they do."

FBI Agent Steve Wollory pledged the agency would follow up on all leads. But the FBI will not investigate the current incidents as the letters contain no threat of violence, Wollory said. Local authorities plan to lead the investigations, he added.

Salam Al-Marayati, president of the Muslim Public Affairs Council, took the podium to call out the letter's anonymous author.

"You are a coward unless you come here and debate the points that you apparently believe so much in," Al-Marayat said.

"Let us debate these issues out in the open and let this bacteria be exposed to oxygen and nutrition and healthy water so that we can remove this infection from America," he added.

Hedab Tarifi, chair of the Islamic Center of Southern California, followed up with a tantamount message:

Don't waste your energy because the law enforcement is on you," Tarifi said. "You're not going to scare us."

Letter sent across the country

In addition to the five California mosques, Wollory said another letter had made its way to the Savannah Islamic Center located about 2,500 miles away in Georgia.

Wollory said the letters look similar in language but stressed he did not want to be absolute.

"They look like they've come from the same author," Wollory said.

In similar past incidents, the FBI has been able to trace it back to the origin, he added.

On Monday, the gates of the Savannah Islamic Center were decorated with flowers and notes from community members expressing solidarity.

The mosque responded with a post on Facebook.

"Thank you," it said.