Other details were not immediately available. LAPD officials scheduled a press conference for 4:30 p.m. Monday.
Investigators believe Ji, a 24-year-old graduate student from China, was walking home from a study group about 12:45 a.m. Thursday when he was assaulted near 29th Street and Orchard Avenue.
Los Angeles police said he was hit with a blunt object but managed to get back to his 30th Street apartment, where his body was discovered later that morning.
University officials said it appeared Ji suffered a head injury.
Detectives determined Ji had been attacked earlier in part because of a "significant amount of blood" they followed from his apartment to the scene of the assault, said an LAPD official familiar with the investigation.
The dried, splattered trail could still be seen Friday, weaving from the sidewalk to the street and back again.
The motive for the attack was unclear.
Smith said Friday that detectives were "pulling out all the stops to get this thing wrapped up."
"It's a very high priority for us to make sure that we take these guys into custody," he said.
Ji's death sent fresh shock waves through USC, a community already sensitive to safety concerns after two Chinese graduate students were shot and killed in 2012 during a botched robbery just blocks away from campus.
Six months later, a man opened fire outside an on-campus Halloween party. Four people were wounded, though none were USC students.
After the 2012 incidents, USC and Los Angeles police amplified resources to the South L.A. campus. The university limited public access to the campus in the evening, the school's Department of Public Safety installed additional security cameras and license plate readers, and dorms introduced fingerprint scanners for entry.
About 1,500 feet of fencing also went up along the school's 2-mile perimeter, and security guards began checking identification cards of everyone coming inside after 9 p.m.
The LAPD devoted 30 officers to a new University Park task force, a team focused primarily on the neighborhoods near USC but that also assists school police with on-campus incidents, according to Smith.
LAPD detectives are also embedded with campus public safety staff, he said, and the agencies attend each other's crime data meetings to stay up-to-date on what is affecting the university and surrounding area.
After the measures were implemented, property crime at the university was cut in half during overnight hours, according to statistics from the university and the LAPD.
Across all hours, violent crimes fell from 11 reported incidents in 2012 to four in 2013.