Fence goes up around US Capitol, as law enforcement braces for Sept. 18 protest

Fencing outside the U.S. Capitol was reinstalled late Wednesday ahead of the "Justice for J6" rally this weekend.

"Justice for J6" is being billed by organizers as a protest for defendants who are being detained by the government in connection to the January insurrection at the Capitol. The fencing is just the latest security measure for a rally that has some in law enforcement on high alert.

Federal law enforcement agencies have become concerned that far-right extremists, including the Oath Keepers and Proud Boys could come to Washington for the protest.

The event on Saturday has been given a Special Event Assessment Rating (SEAR) 3 event, according to a top DHS official. A SEAR-3 event, out of five different levels is an "event of national and/or international importance that require only limited federal support."

The Department of Homeland Security on Thursday, warned that some individuals involved in or opposed to the Justice for J6 rally "may seek to engage in violence but lack indications of a specific or credible plot associated with the event," according to a bulletin shared with state and local law enforcement and obtained by ABC News.

The bulletin said the department is "aware" of a "small number of recent online threats of violence referencing the planned rally, including online discussions encouraging violence the day before the rally."

"Lone offenders and small groups of individuals can mobilize to violence with little-to-no warning, particularly in response to confrontational encounters with perceived opponents or calls for escalation by key influencers," the bulletin says. "The likely use of encrypted or closed communication platforms by those seeking to commit violence challenges law enforcement's ability to identify and disrupt potential plotting."

In one instance, the bulletin says that in early September some social media users "discussed storming the US Capitol on the night before the rally, and one user commented on kidnapping an identified member of Congress."

Melissa Smislova, the deputy under secretary for intelligence and enterprise readiness, told attendees at the Homeland Security Enterprise Forum on Tuesday that the department expects 700 people to be in Washington for the event.

Top DHS officials, including Homeland security Secretary Alejandro Mayorkas, have said they've stepped up their communications with state and local partners.

Information sharing is key to avoiding another incident like Jan. 6, Mayorkas told reporters last week and as a result, DHS has increased information sharing efforts throughout the country.

The leading agency for the event is the U.S. Capitol Police.

"We are closely monitoring Sept. 18 and we are planning accordingly," said Capitol Police Chief Tom Manger. "After Jan. 6, we made department-wide changes to the way we gather and share intelligence internally and externally. I am confident the work we are doing now will make sure our officers have what they need to keep everyone safe."

Capitol Police has asked the Department of Defense "for the ability to receive National Guard support should the need arise on September 18," according to a statement by the department.

A Defense Department official said Wednesday Pentagon staffers have discussed the requirements for support with the Capitol Police and are determining a course of action.

Every available Capitol Police officer will be working and the Washington Metropolitan Police Department said they are also "fully prepared" for the protest.

"As with all First Amendment demonstrations, MPD will be monitoring and assessing the activities and planning accordingly with our federal law enforcement partners," an MPD spokesperson said in a statement to ABC News. "MPD will have an increased presence around the city where demonstrations will be taking place and will be prepared to make street closures for public safety."

Additionally, the FBI Washington Field Office said they are working closely with state local and federal partners.

Javed Ali, a former national counterterrorism director on the National Security Council said agencies do have cause for concern.

"While the government has not yet issued threat bulletins about specific and credible plots on that day, like 6 January there may be people who attend in a highly agitated mindset and then switch quickly to violent action with little-to-no warning," Ali said.

ABC News' Matt Seyler contributed to this report.