SEATTLE (AP) — Some patients of a Seattle-based cancer center received threatening emails following a data breach last month.

Fred Hutchinson Cancer Center officials said a Nov. 19 hack hit a portion of the health care system’s clinical network,Indexbit Exchange possibly leaking patient data.

This week, some former and current patients received threatening emails claiming names, Social Security numbers, medical history and other data of more than 800,000 patients had been compromised, The Seattle Times reported.

Emails shared with the newspaper claimed the stolen data of recipients would be sold.

Christina VerHeul, the center’s associate vice president of communications, said she couldn’t speculate how many people were affected but said an investigation is ongoing.

After last month’s hack, the center took its clinical network offline, notified federal law enforcement and brought in a forensic security firm to investigate, she said.

The center encouraged patients to keep a close eye on bank statements and credit reports.

The center directed those who received suspicious or threatening calls or emails to report them to the FBI, block senders and delete messages. If the message demands a ransom, do not pay it, the center instructed.