Basecamp, maker of the popular project-management app by the same name, was back online Monday afternoon after sustaining a crippling denial-of-service attack earlier in the day that rendered its services unavailable to virtually all users.

In a blog post published Monday, Basecamp officials said the attack began after they spurned a demand to pay an unspecified ransom to avoid a threatened assault on their site. The flood of data that came after the demand was rebuffed peaked at about 20 gigabits per second, preventing legitimate traffic from passing through the site's overwhelmed data connections.

"We've learned that the very same criminals currently attacking and trying to extort us hit others just last week," the Basecamp blog post stated. "We're comparing notes with everyone affected who have been in touch. The blackmail came from an address matching this pattern: dari***@gmail.com. If you have been extorted by this person, please get in contact so we can compare notes on both technical defenses and the law enforcement effort to hunt them down."

Monday's campaign against Basecamp is part of a rash of similar attacks. Three weeks ago, social networking site Meetup also suffered a denial-of-service attack after refusing to pay a $300 ransom that was demanded in an e-mail sent to CEO Scott Heiferman. Last week, social media management system Hootsuite was hit by a data flood that briefly made Web traffic to its dashboard and mobile APIs unavailable for most users.

Extortion-motivated denial-of-service attacks harken back to an earlier era of online crime. More recently, the data floods, at least those that get mentioned in blog and media reports, have largely been an instrument of revenge and protest. It's unclear what has caused the recent uptick in reports of ransom-based denial-of-service attacks.