BOSTON—As investigators continue to probe Monday's deadly bombings at the Boston Marathon here, self-deputized Internet detectives shifted into overdrive on Wednesday—with users of two popular online bulletin boards claiming they had identified the culprit.

On 4chan, an image-driven message board, users zeroed in on a surveillance image showing a man near the marathon's finish line wearing black pants, black shirt and white hat and carrying a black backpack similar to the one recovered near the site of one of the blasts.

In subsequent aerial images, the man is seen without the backpack.

Like a nerdy episode of "CSI," "4channers," as they like to call themselves, proceeded to debate the veracity of the find, even launching a shared Google spreadsheet to organize the armchair intel.

Other 4chan users spotted another man carrying a black backpack whom they deemed suspicious.

Meanwhile, Reddit users were engaged in a similar debate over both photos.

While some fellow users were quick to mock their fellow faux forensics, the FBI and local law enforcement have been asking the public for help, soliciting photos and video that may help solve the crime. Redditors and 4channers are simply following instruction.

