In the chaos following a pair of explosions at the final stretch of the Boston Marathon on Monday, a wild report suddenly appeared: another, possibly related explosion had gone off around 3 p.m. at the John F. Kennedy Library, located on the shore of the Boston Harbor, approximately 3.5 miles southeast of the initial explosions in Copley Square. (Check out the Wire's map to get situated with the geography of today's events.) But the "explosion" later turned out to be an unrelated mechanical fire. So how did word of a third explosion spread?

A quick review of Twitter isolated the first report of an explosion at the library: a publicly available audio feed pulled from a police radio band employed by officers dispatched to downtown Boston. As personnel blanketed the area, an unnamed officer, speaking into his radio, described a "confirmed explosion" at the library's facility:

Despite the off-the-cuff nature of police radios — they're used to organize and deploy personnel, not verify and promulgate news — the unconfirmed comment was quickly picked up:

Shortly thereafter, the director of the JFK Library told the Dorchester Reporter, a local newspaper, that the damage onsite "was confined to a mechanical room near the archive section of the library" and the library's Twitter feed described the damage as a "fire":

Here's what people onscene were looking at:

Within an hour, the Boston Police Department began spreading reports, via statements to the media, that the damage to the library was actually caused by an explosion which may or may not have been linked to the earlier explosions. One press officer for BPD told the Talking Points Memo that "There was another device, confirmed at JFK" that went off around or after 2:55 p.m.; another said the same to the New York Post, which noted that "Boston fire officials previously said that the third explosion was linked to the ones that occurred at the Marathon but later updated their information to say that the explosion was not related."

Then, at a press conference with Governor Deval Patrick at 3:50 p.m., Police Commissioner Edward Davis, based on his department's latest intel, announced that an explosion did take place at the JFK Library — contradicting reports by the library's own staff that the damage was caused by a small fire limited to a room housing the building's HVAC equipment. Davis indicated in one statement that he did not yet know whether the two incidents were related, but that "we are treating them as if they are." Later in the same press conference, Davis said they may be related.

The story soon changed as investigators conclusively determined that an explosion — accidental or planned — had not in fact wrecked the library. At 5:22 p.m., BPD's Twitter feed announced that the damage onscene was limited to a fire:

At 6:23, the library announced on Twitter that the fire had been put out, and that there were no injuries to report:

The Boston Globe soon reported the details of the fire: "The fire started just after 3 p.m. in the vicinity of HVAC equipment in a section of the building opened in 2011 that houses offices, a classroom, and some archival material, said library spokeswoman Rachel Flor. The fire was quickly extinguished." (While it's still unclear how exactly the fire started, it was nothing like the explosions at Copley Square.)

Thus ends — for now — one of the stranger subplots of Monday's events, which took the lives of at least two people and appears to have injured more than 100. On one hand, the concern was understandable: smoke doesn't exactly billow out of libraries every day, and the coincidence was plausible enough, given the number of unexploded devices found lying around downtown Boston. On the other hand, it wasn't exactly clear why a group or a single individual, if wishing to target human beings, would place an explosive device in a library's maintenance room. The open disagreement between two very official sources — the Boston Police Department and the staff of the JFK Library — only added to the chaos that each was trying to dispel.

To keep up with news as it comes in — and the array of reports, both confirmed and unconfirmed — be sure to keep up with our live updates tonight and going forward.