Isaac W. Torres was picked up by authorities in McHenry County, Illinois on an unrelated warrant. Beloit investigators have yet to speak with him.
Police also are seeking Eric Salazar-Mota, 21, and Sergio Ortiz-Raygoza who police believe are no longer in the area.
All three are believed to have various gang ties, Zibolski said. Ramos Sr. may have had past gang affiliations, Zibolski said, but didn't appear to do anything to provoke the attack.
Ziblolski described the three men as "persons of interest," and there could be additional subjects.
Austin Ramos Jr., 5, was shot and killed about 7 p.m. Friday, Jan. 22, near the intersection of Prairie Avenue and Shopiere Road in Beloit.
Ramos was a passenger in the car driven by his father, Austin Sr., when another vehicle pulled alongside and opened fire.
Ramos was shot in the abdomen and died a short time later at Beloit Memorial Hospital of "firearm-related trauma," the Rock County medical examiner's office concluded.
In October, Torres and three other men were charged with a battery where police said the group jumped two men outside a Beloit bar. One of the four, Jose Ortiz, was found later with a fatal gunshot wound, according to court records.
A tip line has been set up at 608-364-6865.