Biden to deliver presidential address amid heightened security

With security fences still surrounding the Capitol and continuing concerns over extremist threats, President Joe Biden's first joint address to Congress will take place under heavy guard -- though details remain murky.

"Heightened security at the U.S. Capitol in advance of President Biden's remarks to the joint session ... underscores the ongoing threat from far-right extremists in the United States now more than three months after the insurrection on 6 January," Javed Ali, the former counterterrorism director on the National Security Council, told ABC News.

Biden's speech comes just months after the deadly Jan. 6 insurrection when pro-trump demonstrators stormed the Capitol in an effort to stop the electoral vote count in Congress and less than a month after a Capitol Police officer was killed when a man rammed his car into a security barricade. Security concerns have also revolved around Biden's speech in particular.

At a House Committee hearing in February, Acting Capitol Police Chief Yogananda Pittman warned of threats to disrupt Biden's speech to a joint session of Congress, which at that time was not yet scheduled.

"We know that members of the militia groups that were present on Jan. 6 have stated their desires that they want to blow up the Capitol and kill as many members as possible with a direct nexus to the State of the Union, which we know that date has not been identified," she testified. "So, based on that information we think that it's prudent that Capitol Police maintain its enhanced and robust security posture until we address those vulnerabilities going forward."

The Department has not elaborated on those threats since her testimony.

While fencing still remains on the perimeter of the Capitol after the Jan. 6 insurrection and presidential inauguration, the U.S. Capitol Police would not comment on security specifics ahead of the president’s speech except to say the Capitol building will be restricted to those with proper credentials.

The D.C. National Guard, for its part, activated as many as 7,000 personnel after the Capitol riot and is leaving the remaining 250 personnel in place near the Capitol to support law enforcement and assist police with street closures "as needed" until May 9., D.C. Mayor Muriel Bowser said at a briefing.

Since Biden’s speech is a National Special Security Event (NSSE), security is coordinated by the Secret Service.

Ali said the NSSE designation means the "highest level of physical protection measures will be implemented involving thousands of law enforcement and Homeland Security personnel across the federal and state and local governments."

On Monday, Washington D.C. Homeland Security and Emergency Management Agency Director Chris Rodriguez told reporters the agency is coordinating with its federal partners and the district’s command center will be operational starting at 4 p.m. on Wednesday. Only 200 people will be allowed to sit inside the House chamber Wednesday evening, a far departure from the usually packed chamber.

House Speaker Nancy Pelosi, however, told reporters Tuesday she was briefed about security ahead of the speech and restrictions regarding attendance spring mostly from COVID-19.

"I wish I had had this briefing before January 6th but we insisted on knowing every detail of it. Actually most of our, shall we say, limitations spring from COVID, not from as much from security in terms of the narrow number of people that's dictated by the physician and the sergeant-at-arms, but the physician mostly," Pelosi said.

The Department of Homeland Security has recently taken steps to prioritize combatting domestic violent extremism.

In January, it issued a warning that anger "fueled by false narratives," especially unfounded claims about the 2020 presidential election, could lead some inside the country to launch attacks in the coming weeks.

Earlier this week, the secretary of Homeland Security launched an internal review of all DHS personnel to weed out any domestic extremists that may be working at the Department.

ABC News' Beatrice Peterson contributed to this report.