The data breach was discovered Thursday, and it was not immediately clear how many employees were affected or when the theft occurred, said Howard Libit, spokesman for Mayor Stephanie Rawlings-Blake.
"We are currently trying to assess the scope," Libit said. "As we determine who has been affected, the city is certainly going to take the appropriate steps in terms of providing credit monitoring and other services."
Baltimore City is the latest agency to be electronically targeted.
The computer system at MedStar Health was attacked two weeks ago, which forced thousands of employees in the state's second-largest health care provider to resort to paper medical records and transactions. Last year it was revealed that hackers had breached networks of the federal Office of Personnel Management and made off with the private data of at least 21 million people.
The FBI and Baltimore police are investigating the theft of city data, Libit said. An email was sent Thursday evening warning all city employees.
"We have not seen it limited to a specific agency of city government," Libit said.
The breach was revealed when a "few dozen" city employees attempted to file tax returns and were rejected, he said.
Officials suspect that information such as Social Security numbers, names and birthdays were stolen. Libit said the Maryland attorney general and Internal Revenue Service were notified.
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