The US Capitol Police chief said Thursday his department was probing its much-criticized response to the storming of the legislative building by supporters of President Donald Trump.

Steven Sund, who heads the agency tasked with protecting the US Congress, said officers had been expecting a rally for demonstrators wishing to exercise their freedom of speech under the US Constitution's First Amendment.

"But make no mistake -- these mass riots were not First Amendment activities; they were criminal riotous behavior," Sund said in a statement.

The Capitol Police's handling of Wednesday's mass break-in has drawn intense scrutiny, with observers wondering why there seemed to be only a small number of officers protecting the famous building.

They were quickly overwhelmed by protesters and waited until it was too late to request backup from the Washington metropolitan police force, which normally does not have authority to work on federal properties like the Capitol.

The Capitol force was "conducting a thorough review of this incident, security planning and policies and procedures," Sund said.

Washington Police Chief Robert Contee said his agency only got a call for help at 1:00 pm Wednesday, well after protesters had begin forcing their way through barriers.

"Things were already pretty bad at that point," Contee said Thursday.

"This was plainly a failure," said Brian Schatz, a Senator from Hawaii.

"US Capitol security needs a total overhaul," he tweeted.

"The physical breaching and desecration of our temple of democracy must never happen again."

A Capitol police officer fatally shot one woman as she tried to enter a corridor in the building.

That officer had been placed on administrative leave while the shooting is investigated, Sund said.

He pointed to a chaotic day with multiple incidents unfolding simultaneously including the discovery of two pipe bombs.

"The violent attack on the US Capitol was unlike any I have ever experienced in my 30 years in law enforcement here in Washington, DC," Sund said.

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