WASHINGTON — Fencing outside U.S. Capitol is expected to return ahead of the "Justice for J6" rally, a source familiar with the plans confirmed to ABC News.

The fencing, erected after the Jan. 6 riot, was removed in July.

"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.

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

"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."

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 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.

Homeland Security Secretary Alejandro Mayorkas told reporters on Wednesday that information sharing is key to avoiding another incident like Jan. 6.

He said the Department of Homeland Security has increased information sharing efforts throughout the country.

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