The lone gunman who killed 13 people and himself at an immigrant center in Binghamton, N.Y., Friday may have been depressed after losing his job and upset about his poor English skills, the city's mayor says.

"He felt that people were looking down on him," Mayor Matthew Ryan says.

The shooting began at around 10:30 a.m. when police say 42-year-old Jiverly Wong, of Johnson City, walked into the American Civic Association after blocking the building's back entrance with his van and opened fire. At least 37 hostages were trapped inside the building.

"We are stricken with grief and share with the victim's families, community and the entire nation," says American Civic Association spokesperson Angela Leach.

Sources say at the time of the incident, the center was conducting an immigration test. The Vietnamese immigrant, who became a U.S. citizen in the 1990s, took English classes at the center only last month.

Authorities believe the rampage was premeditated. Wong was found covered in body armor with an ammunition satchel. Police have searched Wong's home and seized three computer hard drives, a brown canvas rifle case, a briefcase, a small suitcase and several paper bags.