Intruders broke into Arlington County’s employee payroll system recently and were able to alter the paychecks of an undisclosed number of county workers, police said Wednesday.

Police spokeswoman Ashley Savage would not say exactly when the break-in occurred, when it was discovered or how many people were affected, citing an ongoing investigation.

But she said police believe that all those affected have been notified, their pay was restored and the payroll system corrected. It is not known whether any county funds were lost in the attack.

No residents’ files were affected, Savage said.

Since the intrusion was discovered, Arlington County has installed “enhanced security features” to safeguard both email and other critical computing systems, she said in a news release.

The break-in apparently was targeted at individual employees and occurred through a phishing attack, in which someone on the network opens an email or an email attachment from a stranger, launching the attack.