Maryland shooting
Darion Marcus Aguilar, 19, of College Park, Maryland, identified by police as the gunman in Saturday's Columbia Mall shooting, is seen in an undated photo released by the Howard County Police Department in Maryland on January 26, 2014.Reuters

U.S. police officials are yet to find a motive behind the shooting in a mall in Columbia, Maryland, that claimed three lives, including that of the shooter, Darion Marcus Aguilar.

Recent investigations showed that Aguilar, 19, was generally unhappy in the days leading to the shooting, a journal maintained by the shooter revealed. In the journal, Aguilar "does express some general unhappiness with his life, but I really don't have any other information about that now," Howard County Police Chief Bill McMahon told reporters Sunday, CNN reported.

A woman identified as his mother told reporters on Sunday that she too is unaware why her son went on a shooting rampage on Saturday.

"I don't know what happened. I really don't," the woman said, according to ABC News. "You can talk to any of his friends and see what a gentle person he is. He never had a gun before, never interested in guns. I don't know what happened. I just don't know what happened."

According to reports, the 19-year-old bought the 12-gauge shotgun in December. Briefing reporters, police officials said on Sunday that Aguilar entered the the mall in Columbia around 10:15 a.m. on Saturday morning, and he reached the Zumiez store, a skateboarding shop, at around 11:15 a.m.

Shortly, he started firing, killing Brianna Benlolo, 21, of College Park, Md., and Tyler Johnson, 25, of Ellicott City, Md., both employees of the shop. Five others were injured.

Aguilar had graduated from high school last year and was working at a Dunkin' Donuts store. He had no criminal record, and police officials have not been able to find a relationship between him and his victims.

The mall has been shut down since the shooting, and it will reopen only on Monday at 1 p.m. local time.