LOS ANGELES >> Four teenagers were arrested in Thursday’s deadly attack on a University of Southern California graduate student as he walked home from a study group, with police saying the incident was one of at least two robberies the group attempted that day.

Xinran Ji, a 24-year-old graduate student from China, was allegedly hit on the head with a baseball bat by at least one of the suspects, according to two law enforcement sources. Detectives are probing whether other weapons were also used, the sources said. The suspects range in age from 16 to 19.

"He was literally beaten to death," said LAPD Cmdr. Andrew Smith. "I don’t know why a group of young people would go on a crime spree as terrible as this."

Despite his injury, Ji managed to make it back to his 30th Street apartment, where his body was found around 7 a.m. by a roommate. Detectives followed a bloody trail from his home to where they believe he was attacked, near 29th Street and Orchard Avenue.

The two adult suspects were identified by police as Jonathan DelCarmen, 19, and Andrew Garcia, 18. Both were arrested on suspicion of homicide, but Garcia faces additional allegations of assault with a deadly weapon and robbery.

Two juveniles, a 17-year-old male and 16-year-old female, were also arrested on suspicion of murder, assault with a deadly weapon and robbery. Their names were not released by police.

Prosecutors were expected to file charges Tuesday, LAPD officials said. Smith said the murder charges would carry special circumstance allegations that would make the suspects eligible for the death penalty.

Sources familiar with the investigation said detectives believe the suspects used a baseball bat in the attack on Ji. Police said there were no indications the suspects were gang members or that the attack was racially motivated.

"It was a senseless act of violence, regardless of their rap sheets. It was a senseless act," said LAPD Cmdr. Bill Scott. "I don’t know what was going on in these individuals’ minds."

A fifth suspect, a 14-year-old girl, was also in custody regarding a second incident, which police described as a robbery that took place at Dockweiler Beach "a few hours" after Ji was beaten near the South L.A. campus about 12:45 a.m.

Two victims in the Dockweiler incident flagged down police who detained two of the suspects about 3 a.m., Smith said. Detectives then linked them to the earlier attack at USC.

Detectives said the 14-year-old was involved in Ji’s assault "in some manner," Smith said, but that the "detail and depth of her involvement" was not clear.

A source familiar with the investigation said footage from a license plate reader near USC helped lead them to the suspects.

Investigators are also trying to determine whether the suspects committed more crimes during Thursday night or early Friday morning, Smith said. Anyone with information about possible incidents was asked to contact police.

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 was a USC student.

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.