Here are the FBI's two wanted posters:

Again, the investigators are referring to these two people as suspects — not persons of interest. DesLauriers, the special agent in charge, said at the briefing:

After a very detailed analysis of photo, video and other evidence, we are releasing photos of these two suspects. They are identified as suspect 1 and suspect 2. They appear to be associated. Suspect 1 is wearing a dark hat, suspect 2 is wearing a white hat. Suspect 2 set down a backpack at the site of the second explosion. ... “As you can see from one of the images, suspects 1 and 2 appear to be walking together through the marathon crowd on Boylston Street in the direction of the finish line. That image was captured as they walked on Boylston in the vicinity of the intersection with Gloucester Street. As you can see the quality of the photos is quite good. We will continue to work on developing additional images to improve their identification value.

Here's how the rest of Day Four of the Boston Marathon aftermath played out:

The Delay of the Big Reveal

As Day Four of the Boston Marathon bombing investigation began, several outlets were reporting that the FBI-led investigation was on the verge of releasing what they believed to be photos of two men involved with the bombings. The Boston Globe reported that the FBI had obtained video stills of "two separate suspects in the Boston Marathon bombings carrying black bags at each explosion site and are planning to release the images today in an appeal for the public's help in identifying the men, according to an official briefed on the case." In a Congressional hearing today, Homeland Security Secretary Janet Napolitano tempered that lead, stating she "wouldn't characterize them as 'suspects' under the technical term. But we need the public's help in locating these individuals." And she said, perhaps explaining the complexity of forensic analysis still being done, that "there's lots and lots of video. There is some video that has raised the question of those that the FBI would like to speak with." So what have we been waiting for?

The plan sounded simple enough: The FBI needs the public's help, and planned to release photos to help put names and bodies to photos and video And you'd think if these suspects and/or people of interest were very important and/or dangerous, you'd want to get that information out sooner rather than later, right? So why is the conference only happening at 5 p.m.? Slate spoke to an expert:

Gregory D. Lee, a longtime supervisory special agent for the DEA and now a criminal justice consultant, notes that it’s possible the video footage isn’t the FBI’s best or only lead. "To me, there’s something behind the scenes that they're working on," says Lee, noting that the agency might be waiting to see whether these other leads pan out before deciding to release the footage.

And he could be right: A new lead might mean the investigation veering off and needing more time to confirm that lead. But according to Reuters, the FBI also seems to be trying to figure out whether or not releasing these photos is the right thing to do: "There is an intense debate inside the government as to whether such photographs should be officially released and the public should be asked for help identifying people in them."