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. Three juveniles were arrested Wednesday in connection to the attack, Officer Tanya Little of the Philadelphia Police Department told FoxNews.com.

Similar attacks are also being investigated in cities likes Los Angeles, Chicago, Cleveland, Washington, D.C., and Milwaukee, where 30 people were arrested after alleged mob attacks erupted at the Wisconsin State Fair on Aug. 4. At least 18 people were injured in or around the grounds, including seven police officers, authorities said.

Milwaukee Police Chief Ed Flynn called the attacks "reprehensible," and some fairgoers, including Norb Roffers, said it appeared bystanders were targeted because of their race.

"It looked like they were just going after white guys, white people," Roffers told Wisconsin’s Newsradio 620.

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.

Without a confession, a witness account or some epithet overheard by the victim, no charges will be filed related to ethnic intimidation or a hate crime, Ross said.

“If we don’t know and can’t prove it, we can’t charge it,” he said. “It’s just a very difficult charge to prove … We’re in the business of what we can prove, not what we think.”

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.

West Allis Police Chief Mike Jungbluth said the victims, who ranged in age from “young all the way up to adults,” suffered broken bones in the alleged attacks.

“They were targeting anyone who was white or appeared to look white … We are actively looking at violations of hate crime statues in Wisconsin,” 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.”

SLIDESHOW: Click here for more on the alleged "flash mob" attacks from MyFoxPhilly.com