CHICAGO -- The head of the FBI has a theory about what is behind a rise in gun violence in big cities. And he talked about it on Monday in one of those cities -- Chicago.

Six people died and 28 were wounded in gun violence over the weekend in Chicago. Homicides in Chicago are up 19 percent this year. In other major cities there have been sharper increases in homicides. Homicides are up 44 percent in Washington D.C., 62 percent in Milwaukee, and 73 percent in Nashville.

FBI Director James Comey discussed the spike with a group of police chiefs on Monday.

FBI Director James Comey. CBS News

"Some part of what is going on is likely a chill wind that has blown through law enforcement over the last year," he said. "That wind is made up of a whole series of viral videos and the public outcry that followed them."

Comey believes cellphone video posted outlined from Freddie Gray's arrest in Baltimore, and Eric Garner's in New York, has put police officers to the test and could be changing how they work. Both Gray and Garner died after their encounters with police.

"In today's YouTube world, are officers reluctant to get out of their cars and do the work that reduces violent crime?" Comey asked.

CBS News

But Comey's remarks put him at odds with President Obama.

"The available evidence at this point does not support the notion that law enforcement officers across the country are shying away from fulfilling their responsibilities," said White House Spokesman Josh Earnest.

Comey said he has no data on the YouTube effect, but said his hunch was strong enough to start talking about it publicly. Others say the rise in crime is connected to drug-related violence fueled by increased gang activity.