Chicago police say they have not arrested anyone in connection with this weekend's violence in the city, in which 66 people were shot, 12 of them fatally.The city's police chief, Superintendent Eddie Johnson, said additional officers have been deployed to the neighborhoods where the violence took place to prevent retaliatory shootings. He told reporters the extra police presence has led to the arrests of 46 people on firearms charges. Johnson said his officers are following promising leads in the investigation into the weekend's violence. Police have blamed much of the bloodshed on gangs.The deaths took place in the span of just 24 hours, one of the worst shooting sprees the city has seen.At one point, 40 people were shot in the span of seven hours, according to a tally compiled by the Chicago Tribune. The dead included a 17-year-old girl who was shot in the face.Police say gang members use large summer crowds as a cover in some cases, firing indiscriminately into groups in rival territories.Last week, hundreds of demonstrators blocked a major Chicago highway to protest gun violence and demand Mayor Rahm Emanuel resign.On Monday, Emanuel urged people to report those they believe were responsible for the weekend shootings. "They have a moral responsibility to speak up," he said.President Donald Trump's lawyer, former New York City mayor Rudy Giuliani, criticized Emanuel on Monday and urged people to vote against him in next year's elections.Chicago police blame the gun violence on a large number of weapons on the streets, as well as what they say is the belief by gang members that they can gun down a rival and get away with it. They also criticize short jail sentences that are sometimes given to repeat gun offenders.More than 300 people have been shot to death in Chicago this year, the most of any U.S. city. That is more than the country's two largest cities — New York and Los Angeles.However, Chicago police say shooting deaths are down by 25 percent from the same time last year, and say 5,500 illegal guns have been confiscated since January.