(CNN) -- In the aftermath of dramatic events like Monday's bombing attack at the Boston Marathon, it's a truth of our times that millions of people will get early bits of news via social media.

To be sure, sites like Twitter and Facebook were used extensively by police, relief groups and governments to share important information about the bombings. But there's also a more unfortunate side to how the Web responds to sudden bad news.

Sometimes accidentally and sometimes maliciously, false information gets loose. And in the rapid-fire digital echo chamber, it doesn't take long to spread.

"On days like this, Twitter shows its best & worst: loads of info at huge speed, but often false & sometimes deliberately so," said Mark Blank-Settle, of the BBC College of Journalism, in a post on the site.

As always, news discovered online (or anywhere else, really) should be double-checked before it's passed along -- especially in times of tragedy.

The news comes on a day when authorities have made "significant progress" in the case but no arrests, a federal law enforcement source told CNN's John King. Sources previously told CNN that a suspect was in custody, but both Boston police and the FBI denied that any arrests had been made.