The tapes were lost by the storage firm during a shipment to one of its storage facilities, according to statement Time Warner released Monday.
The Secret Service is investigating the case but has not turned up any evidence that the tapes or their contents have been accessed, the media giant said.
The missing tapes contained names and Social Security numbers of current and former U.S.-based employees, some of their dependents and service providers of Time Warner and its affiliates. The company was sending out notices to those whose data may have been on the tapes. Published reports indicate that 600,000 people were affected. No customer data was included on the tapes.
Larry Cockell, Time Warner's senior vice president and CSO, said the company regrets the incident and is working closely with law enforcement on the case.
"While we have no evidence to suggest the information on the tapes has been accessed or misued, we are providing current and former employeees with resources to monitor their credit reports while our investigation continues," he said in a statement.
Time Warner said it has used the data-storage company for several years to ship and store its back-up tapes offsite, for data security purposes.