Disgruntled employees and unhappy ex-workers pose a “significant” threat to companies’ computer networks and sensitive data, federal officials are warning.

In a public notice this week, the FBI and Department of Homeland Security warned, “disgruntled and former employees pose a significant cyber threat to U.S. businesses due to their authorized access to sensitive information and the networks businesses rely on.” 

{mosads}The FBI has launched “several significant” investigations into cases where disgruntled workers tried to destroy companies’ data, steal customer information or nab industry secrets, the agencies said.

Officials have also responded to multiple cases where an upset or former employee essentially held a company hostage by threatening to block their access to their website or disable key functions.

Cyberattacks by current or former employees can cost companies millions of dollars, the agencies added.

Officials are still reeling from the massive data breach at Home Depot, which led to the theft of 56 million credit and debit card numbers.

Recent hacks of JPMorgan Chase and sandwich chain Jimmy John’s, and the theft of hundreds of nude and intimate photos of celebrities from Apple’s iCloud have raised broader public concerns.

Congress has yet to take action, though the retail and financial industries have tried to mount a response of their own.

In their notice, the federal agencies told companies to regularly review which employees have access to key networks and cut workers off after they leave.