Police departments in several cities around the country are investigating what appear to be incidents of “flash mob”-generated violence, in which packs of dozens or even hundreds of youths appear seemingly out of nowhere to commit assaults, robberies and other crimes against innocent bystanders.

The motive and circumstances surrounding the attacks that have resulted in numerous arrests around the country are being investigated–and law enforcement officials in at least one city are looking into a possible racial component to the crimes.

Philadelphia Mayor Michael Nutter, who recently imposed a stricter curfew in response to the city’s latest attack, addressed black youths directly from the pulpit of his church on Sunday, reportedly saying, “You have damaged your own race.”

“If you want …anybody else to respect you and not be afraid when they see you walking down the street, then leave the innocent people who are walking down the street minding their own damn business. Leave them alone,” Nutter told a mostly black congregation at Mount Carmel Baptist Church in West Philadelphia, according to the Philadelphia Inquirer.

Nutter, the city’s third black mayor, spoke after a mob attack July 29 in downtown Philadelphia during which 20 to 30 black youths allegedly beat and robbed innocent bystanders, according to law enforcement. Several people were injured, including one man who was reportedly hospitalized for a fractured skull, and police arrested four people.

The incident happened the same day police said a group of all-black teens beat a man on a street in Philadelphia’s Old City section– which was caught on surveillance video obtained by Fox affiliate WTXF-TV. {snip}

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But while some witness accounts suggest the attacks are race-based, law enforcement officials say they have no evidence to prove it.

There was “no confession or anything else” to suggest the July 29 attacks in Philadelphia were “racially motivated,” Philadelphia Police Department First Deputy Commissioner Richard Ross told FoxNews.com.

“You can’t just simply look at the race of the offender and the race of the victim and say it’s ethnic intimidation. It may be, but we’re not sure. Does it give us pause? Yes it does,” Ross said.

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Authorities in Wisconsin, meanwhile, say they are investigating 11 new allegations of race-based assaults near the state fairgrounds in which the alleged perpetrators were all African-American and the victims were either white or Hispanic.

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“They were targeting anyone who was white or appeared to look white … We are actively looking at violations of hate crime statutes in Wisconsin,” [West Allis Police Chief Mike] Jungbluth told FoxNews.com, adding that the alleged attacks were “an outrageous occurrence at an event that is truly designed to be about the family.”

[Editor’s Note: Be sure to click through to the original article to watch a video of one of the Philadelphia attacks.]