Last week we saw social media at its worst: arrogant, reckless and even racist.

Dozens of users on the social news forum Reddit resolved to launch their own public investigation into the Boston Marathon bombings by analyzing reams of publicly available photos of the attack sites. It’s called crowdsourcing, and it was a dismal failure.

The bizarre mess of finger-pointing was largely confined to a subreddit, or forum, titled Find Boston Bombers.

Created by a 23-year-old British man who never set foot on the marathon route, the forum soon spun out of control.

All the “suspects” were innocent bystanders.

They had their faces enlarged, circled in red and were given glib nicknames. There was “blue robe guy” and “green hat man” and “roof man.” Users singled out people in photos for holding their backpacks too tightly or looking too serious. False suspects were pointed to because of their race. And some, for all we know, were among the more than 170 victims of the blasts.

It’s as if the tragic whirlwind of events in the last week unfolded in two universes.

In one, cops acted methodically, sifting through evidence and hunting down suspected terrorists in some of the finest police work we’ve seen. In another, self-proclaimed cyber sleuths wasted their time and ours by accusing people with a scant amount of evidence.

The low point for Reddit’s gumshoes came in the wee hours of Friday’s manhunt, when a member claimed to have heard compelling chatter on a police scanner. But the information was wrong and cruel: that missing Brown University student Sunil Tripathi, 22 — who hasn’t been seen since March — was a suspect in the bombings. Worse still: Some media outlets ran with this despicable smear.

Moderation on Reddit is infrequent.

Usually that’s OK because the stakes aren’t this high. But this past week the stakes couldn’t have been higher, especially for Boston.

This chaotic digital search could have easily spawned vigilantes who were thirsty for justice.

And what if being falsely fingered as a suspect sent someone over the edge?

Imagine witnessing unspeakable violence, only to have your photos being circulated as the perpetrator.

Reddit was lucky it didn’t incite further suffering. It’s lucky Bostonians are smarter than that.

So let’s quit while we’re ahead. No more crowdsourced “investigations.”