Shares in Okta Inc. fell 9 percent on Wednesday after the U.S. digital authentication firm said hundreds of its customers may have been affected by a security breach involving hacking group Lapsus$.

The breach sparked concern as the cyber extortion gang had posted what appeared to be internal screenshots from within the organization’s network roughly a day ago.

Okta’s Chief Security Officer David Bradbury said in a series of blog posts that the “maximum potential impact” was to 366 customers whose data was accessed by an outside contractor.

The contractor, Sitel Group, employed an engineer whose laptop the hackers had hijacked, Bradbury said, adding that the 366 figure represented a “worst case scenario” and that the hackers had been constrained in their range of possible actions.

A representative for Sykes, a subsidiary of the Sitel Group, said in an emailed statement that the company was unable to comment on its relationship to its customers but it undertook an “immediate and comprehensive” investigation into the breach and had since determined there was no longer a security risk.

San Francisco-based Okta helps employees of more than 15,000 organizations securely access their networks and applications, so a breach could have serious consequences.

Bradbury said the intruders would have been unable to perform actions such as downloading customer databases or accessing Okta’s source code.

Okta first got wind of the breach in January, he added, while Miami-based Sitel Group only received a forensic report about the incident on March 10, giving Okta a summary of the findings a week later.

Bradbury said he was “greatly disappointed by the long period of time that transpired between our notification to Sitel and the issuance of the complete investigation report.”

The hack—and Okta’s reaction to it—has made some investors nervous. The 10.74 percent fall in share price was the worst one-day percentage drop since 2018, and Raymond James Equity Research downgraded the stock from “strong buy” to “market perform,” in part citing Okta’s handling of the incident.

(Reporting by Raphael Satter; Editing by Shri Navaratnam, Bernadette Baum and Alexander Smith)