Darren Wilson, as you know, was the cop who shot 18-year-old Michael Brown in Missouri. The FBI tracked down the commenter's location using his account, and then scoped out Perez's residence for a day before the agents obtained a search warrant. They determined that he was the one who posted the comment after looking through his computer, where they also found search terms like "Kill Barack Obama" and "Hunt Darren Wilson's Family."
To be clear, Perez was arrested without incident: He admitted to posting those threats, but claimed that he never intended to follow through. According to the US Attorney General's office, authorities figured that he knew law enforcement agents would eventually read the post, and he intentionally wanted to scare them. US Attorney General John Walsh also said that people other than Perez posting comments online threatening to kill police offers should expect to get "some very serious attention" from authorities.
This isn't the first time Google's reports to authorities have led to an arrest, though. Back in August, the company's child-porn detecting technology spotted incriminating images in a user's Gmail account. People were worried that meant the tech juggernaut was looking through everyone's emails, but Google maintained that an automated system was responsible for the discovery, and it couldn't actually parse users' conversations.