COLUMBIA, S.C. — Three men were charged with murder and a fourth was being sought Monday, two days after a Clemson University student was shot and killed by masked intruders who burst into his off-campus home.

The armed invaders broke into the home of 23-year-old Steven Grich on Saturday night to rob his roommate of what they thought would be a big stash of marijuana, the Pickens County Sheriff’s Office said in a statement.

None of the other six people who were there was seriously injured, the statement said.

Sheriff’s investigators arrested 19-year-old Jordan Dalton of Central; 24-year-old Jaron Dalton of Six Mile; and 19-year-old Bernard Ramsey of Central on charges of murder, armed robbery, first-degree burglary and weapons possession. They issued warrants for the arrest of Lester Mosley, 18, of Central, on the same charges.

Grich, McKinley and four others were inside the home when three men entered through an unlocked back door, authorities said. Only a few grams of marijuana were taken during the robbery. Deputies charged Grich’s 29-year-old roommate, Robert McKinley, with simple possession of marijuana after deputies searching the home found the drugs.

A woman answering the phone at a listing for McKinley on Monday said he was devastated by his roommate’s death and was cooperating with investigators. The woman identified herself as McKinley’s relative but refused to give her name.

Grich was a junior electrical engineering major who transferred to Clemson in fall 2011, school spokeswoman Robin Denny said. He was from Fort Mill. A person who answered the telephone at his home declined to speak with a reporter.

McKinley and Grich lived in Chimney Ridge, an upscale subdivision of more than 100 two-bedroom cottages and four-bedroom homes marketed to Clemson students about two miles from campus. The website for the subdivision touts a 24-hour security-alarm system in every home.

J.J. Andrighetti, a lawyer for the owners of the complex, said each home gets its own security code for the gate. Since the shooting, he said, the community is suggesting residents come by to have the codes changed and share them only with people they trust.

In addition, Chimney Ridge shared video footage from its security cameras with police and has hired a security guard to work from 6 p.m. to 6 a.m., Andrighetti said.

“We’re doing everything we can to help the police and get this solved quickly,” he said.

University officials sent an email to students Sunday reminding them of safety rules such as locking doors, not answering if a stranger knocks and reporting any suspicious behavior. They also made counselors available for Grich’s roommates, neighbors and friends.

“We are so sad about this sudden loss of one of our students,” Dean of Students Joy Smith said in an email. “The death of a young person is always a tragedy, but it is especially so at this time of year. We will support his family and friends and other students during this difficult time.”