Chicago police are warning the public about robbers who have been threatening people with a semiautomatic handgun in Northwest Side neighborhoods.Police said the robbers walk up to victims who are alone and carrying a bag or purse, pull out a black semiautomatic handgun and demand the victim’s belongings.The robberies have been occurring for more than a month in Belmont Cragin, Austin, Albany Park, Logan Square, Avondale, Norwood Park and Hermosa.The robbers are described as a group of about four individuals (men and women) between the ages of 17 and 30. They are about 5 feet 5 to 5 feet 10 and weigh between 100 and 200 pounds. The suspects also sometimes wear black ski masks or surgical masks, investigators say.These are the times and locations of the robberies:4000 block of North Moody Avenue on September 11 at 7:28 p.m.4900 block of North Rutherford Avenue on September 11 at 7:42 p.m.4600 block of North Kedzie Avenue on September 12 at 12:35 p.m.3800 block of West Diversey Avenue on September 25 at 4:11 a.m.2300 block of North Kostner Avenue on September 25 at 4:17 a.m.6200 block of West North Avenue on October 8 at 10:34 a.m.4400 block of West Fullerton Avenue on October 10 at 5:13 a.m.3500 block of North Hamlin Avenue on October 10 at 5:25 a.m.6800 block of West Diversey Avenue on October 10 at 6:44 a.m.2700 block of North Montclare Avenue on October 10 at 6:45 a.m.2100 block of North Latrobe Avenue on October 10 at 7 a.m.2700 block of North Normandy Avenue on October 10 at 7 a.m.5000 block of West Belden Avenue on October 10 at 7:04 a.m.3700 block of West Fullerton Avenue on October 11 at 12:36 p.m.