Police say an automatic weapon was used in the shooting.
Tuesday's arrest came as Mayor Daley renewed his call for tougher federal and state gun laws.
The mayor's message is that stricter gun laws could prevent violence like that which killed three teenagers last Friday. He confirmed there is a suspect in custody. And police tell ABC7 they are still looking for two others in connection with that shooting.
Based on shell casings at the scene investigators say they believe whoever opened fire Friday at 87th and Exchange, killing three teenagers, used an assault rifle. It was a busy afternoon in broad daylight. The shots fired from a moving vehicle and there were apparently several witnesses who have helped police identify at least one suspect. They arrested that person on Monday.
"They have a person in custody based on an arrest warrant already in connection triple homicide this weekend," said Daley.
Police say a judge signed the arrest warrant for the suspect who they describe as a young adult male over the age of 18. He's believed to be a gang member.
The hail of gunfire killed 13-year-old Johnny Edwards, 15-year-old Raheem Washington and 17-year-old Kendrick Pitts. While there are two police surveillance cameras posted nearby to the scene... The one with the best vantage point offers no help.
"The pod at 87th and Exchange was checked?there was a malfunction on it," said Daley.
News of the arrest came during a news conference with the mayor offering a gun control proposal. Police display a table with dozens and dozens of guns they have taken off the streets recently. The collection includes numerous assault weapons.
The mayor's proposal would among other things limit gun purchases and ban assault weapons. Parents of several victims of gun violence in the city are standing with the mayor.
"This year I celebrated my son's 21st birthday at the cemetery with my wife. That's not where I should have been celebrating," said Tom Bosely whose son was killed in 2006.
It is already illegal to own a gun in the city of Chicago. The mayor's proposals are for state and federal gun control laws. As for the South Chicago murders police say they still have no motive for what might have led to the shooting spree.