CWT, the US-based travel management company, paid $4.5 million in Bitcoin in ransom to hackers who stole extensive amounts of sensitive corporate files and said they had knocked 30,000 computers offline, according to a report on the ransom negotiations by Reuters. CWT, which posted revenues of $1.5 billion last year and says it represents more than a third of companies on the S&P 500 U.S. stock index, confirmed the attack but declined to comment on the details of what it said was an ongoing investigation. "We can confirm that after temporarily shutting down our systems as a precautionary measure, our systems are back online and the incident has now ceased," it said in a statement. "While the investigation is at an early stage, we have no indication that personally identifiable information/customer and traveler information has been compromised." CWT said it had immediately informed U.S. law enforcement and European data protection authorities.