Marc Benioff, the Salesforce CEO and co-founder, is facing growing criticism over a joke he made about US Immigration and Customs Enforcement (ICE) monitoring international employees during an event for the company.

Employees publicly expressed outrage about Benioff’s comments in his keynote address at an internal conference in Las Vegas on Tuesday, and leaders at Slack, which Salesforce owns, have said he should apologize.

Benioff reportedly asked international employees to stand, before joking that ICE agents were present and monitoring them, drawing boos from the crowd, several employees told media outlets including Wired and 404Media. A recording of his speech published on an internal Salesforce site was edited to cut his remarks about ICE, Business Insider reported.

The incident comes as hundreds of Salesforce workers reportedly plan to urge Benioff to denounce ICE and cancel future business with the agency, Wired and CNBC reported.