United Nations experts investigating violations of sanctions on North Korea have suffered a “sustained” cyber attack by unknown hackers with “very detailed insight” into their work, according to an email warning seen by Reuters on Monday.

The hackers eventually breached the computer of one of the experts on May 8, the chair of the panel of experts wrote in an email to U.N. officials and the U.N. Security Council’s North Korea sanctions committee, known as the 1718 committee.

“The zip file was sent with a highly personalized message which shows the hackers have very detailed insight into the panel’s current investigations structure and working methods,” read the email, which was sent on May 8.

“As a number of 1718 committee members were targeted in a similar fashion in 2016, I am writing to you all to alert you to this heightened risk,” the chair of the panel of experts wrote, describing the attack as part of a “sustained cyber campaign.”

A spokesman for the Italian mission to the United Nations, which chairs the 1718 sanctions committee, said on Friday that a member of the panel of experts had been hacked.

No further details who might be responsible were immediately available.

Read the whole story from Reuters.