The FBI, NYPD and the Multi-State Information Sharing and Analysis Center are investigating a malicious attack against the city's email system that left all agencies unable to receive email last week. View Full Caption DNAinfo/Colby Hamilton

NEW YORK CITY—The FBI, NYPD and the Multi-State Information Sharing and Analysis Center are investigating a hacking attack against the city's email system that left all agencies unable to receive some messages last week.

According to a City Hall source, the "universal" denial of service attack had been slowed but there was still "ongoing malicious activity" as recently as Monday.

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Some city agencies such as the Department of Transportation set up temporary Gmail accounts to receive emails.

Jackie Albano, a spokeswoman for the city's Department of Information Technology and Telecommunications, said the issue, which started last Tuesday, had been resolved last week. Efforts taken to mitigate the attack may have contributed to slowed emails this week.

While inbound and outbound emails were affected, intra-agency emails were not, meaning that the work of the city could continue.

No sensitive information or data was compromised during the attack, said Albano who characterized it as a "big attack" but downplayed its impact.

"It is a big deal but....it's like a lot of mosquitoes buzzing around you," said Albano.

"The nature of the attack is only designed to interfere with service, not to steal or access any private information. It's designed to slow down email. On the scale of cyber incidences it's kind of low."

But the incident is still being investigated by the three agencies as a matter of course. It's unclear who initiated the attack or why.

"This is standard protocol," said Albano. "The reason why we are sharing information about the attack with these agencies is so that we have a pool of knowledge."

The FBI did not immediately respond to request for comment nor did MS-ISAC.