Here we go again? Reporters inside Capitol Hill have been warned to stay away from doors and windows and part of the complex has been evacuated. The entire complex is now on lockdown due to an “external security threat,” which might be under a nearby bridge or underpass:

DEVELOPING: Message sent to staff in the U.S. Capitol:

"All buildings within the Capitol Complex: External security threat, no entry or exit is permitted, stay away from exterior windows, doors. If outside, seek cover." https://t.co/tqeCpjbXi6

— NBC News (@NBCNews) January 18, 2021

Something suspicious might have been spotted at the interstate highway that passes the complex:

No entry or exit from the U.S. Capitol due to an “external security threat located under the bridge on I-295 at First and F Streets SE,” per note just sent out from the Capitol Police. pic.twitter.com/1vtTVdbgOa

— Eli Yokley (@eyokley) January 18, 2021

This doesn’t appear related to any potential mob action, which would be tough to pull off at this point with the National Guard on duty and fences around the complex. It also doesn’t appear related to a fire located in the area, although the smoke apparently unnerved some inside:

The juxtaposition of the smoke and the security alert at the Capitol was unsettling. https://t.co/vqboPoFjIE

— Olivier Knox (@OKnox) January 18, 2021

We’ll have more as this develops. However, it’s worth noting that the FBI might not be entirely comfortable with the security arrangements for the inauguration. They have begun vetting the National Guard troops in advance of Wednesday’s events for extremist ties, although they have no evidence of any trouble yet:

U.S. defense officials say they are worried about an insider attack or other threat from service members involved in securing President-elect Joe Biden’s inauguration, prompting the FBI to vet all of the 25,000 National Guard troops coming into Washington for the event.

The massive undertaking reflects the extraordinary security concerns that have gripped Washington following the deadly Jan. 6 insurrection at the U.S. Capitol by pro-Trump rioters. And it underscores fears that some of the very people assigned to protect the city over the next several days could present a threat to the incoming president and other VIPs in attendance.

Army Secretary Ryan McCarthy told The Associated Press on Sunday that officials are conscious of the potential threat, and he warned commanders to be on the lookout for any problems within their ranks as the inauguration approaches. So far, however, he and other leaders say they have seen no evidence of any threats, and officials said the vetting hadn’t flagged any issues that they were aware of.

In other words, this additional vetting is evidence of an abundance of caution, not of an actual known problem. The security alert might be something different. Stay tuned.

Update: CBS News has the first video report:

BREAKING: The U.S. Capitol is under lockdown due to an exterior security threat. @nancycordes reports what's happening on CBSN: https://t.co/Nj065CIsxp pic.twitter.com/7JS85rCLAD

— CBS News (@CBSNews) January 18, 2021

They don’t have many other details yet, other than reporters were ordered back to their media trailers. Police then ordered them out of the trailers and to a “more secure location.”

Update: NBC’s Pete Williams reports that it actually was a fire in a homeless encampment that triggered the security alert. The AP also reports the same thing:

Capitol Hill evacuation was prompted by what turned out to be a fire at a nearby homeless encampment, a law enforcement official says – @PeteWilliamsNBC

— NBC News (@NBCNews) January 18, 2021

WASHINGTON (AP) — Officials say evacuation at inaugural rehearsal was due to a fire several blocks away and not believed to be a threat.

— Zeke Miller (@ZekeJMiller) January 18, 2021

This could still be a diversion, so expect the lockdown to continue for a while. The two pipe bombs found outside the RNC and DNC offices on January 6th were likely a diversion too, and the Capitol Hill police aren’t going to take anything for granted.

Update: Here’s more on the fire:

Things seem to be OK at the Capitol. DC Fire's Vito Maggiolo says the incident involved a small fire involving tent that is now out. DC fire was dispatched at 10:14 and the fire was put out. He said the response was "very minimal" and it was "pretty much a non-incident."

— Manu Raju (@mkraju) January 18, 2021

If that’s all this was, perhaps next time it would just be best to alert outside security personnel first.

Update: This makes the security lockdown look a bit more sensible. It wasn’t the fire, Politico’s Lara Seligman reports, but an explosion caused by a propane tank in the camp:

BREAKING: Guard source tells me a propane tank exploded at a homeless camp near the Capitol. Several other propane tanks have been spotted laying around and are being investigated by police.

— Lara Seligman (@laraseligman) January 18, 2021

Maybe DC and Capitol Hill police might consider relocating the camp for the next few days?