Online hackers, who targeted the Massachusetts unemployment offices, used malware to steal personal information of up to 210,000 unemployed residents, according to the state agency.

The Massachusetts Executive Office of Labor and Workforce Development (EOLWD) said on Wednesday that up to 1,500 computers in the Departments of Unemployment Assistance and Career Services were affected by a malware starting April 20. Systems in the mobile One Stop Career Centers were also targeted in the attack.

The department started working with security firm, Symantec to remove the malware known as W32. QAKBOT. It came to know that the on May 16 that the infection had not been cleaned completely, as it was thought before.

"I apologize to our customers and recognize that this is an unwanted problem,” Joanne F. Goldstein, Secretary of Labor and Workforce Development said in a statement.

Symantec’s Security Response page says that W32. QAKBOT is a worm that spreads through network drives and removable drives. After a system is infected, the worm can download additional files, steal information and allow hackers to download more worms on the compromised machine.

The worn also has a rootkit that allows it to hide its presence and it works slowly to avoid detection. The worn might have accessed names, Social Security numbers, Employer Identification Numbers, email addresses, and residential or business addresses of residents in the state.