Answers sought: Was the US Capitol attack planned?

Attackers in organised columns, a woman shouting instructions on a megaphone, and suspicious tours the day before: investigators are probing the possibility that the January 6 attack on the US Capitol was planned, with help from insiders.

Disturbing videos, photos and online communications point to potential conspiracy.

In one video, more than a dozen men wearing assault force-type garb push up the Capitol steps in a line, cutting through the dense crowd toward the building's doors.

In another, a woman in a pink hat gives directions via megaphone to others inside the building, telling them where to go.

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And several men, including two who made it inside the Senate chamber, carried zip ties that could be used to restrain hostages.

Some officials said many of the pro-Trump protestors who broke into the offices of top legislators like House Speaker Nancy Pelosi appeared to know how to navigate the maze-like Capitol.

"They knew where to go," senior Democrat James Clyburn told CBS News. "Yes, somebody on the inside of those buildings were complicit in this."

Prosecutors have arrested dozens so far and say more than 200 could be charged. But they have not used the words "conspiracy" or "plot" to describe the unprecedented assault on the US legislature.

Michael Sherwin, the Washington federal prosecutor overseeing the investigation, said Friday they see "breadcrumbs of organisation" including communications between those inside the building and outside.

He said it was a "Tier 1 top priority" for law enforcement to determine if there was an "overarching command and control" and "organised teams" in the breach of the Capitol.

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"This is going to take weeks if not months to find out the actual motivations of some of these groups," he said.

But, he added, "There is no direct evidence at this point of kill-capture teams and assassinations," he said.

Analysts say much of the activity on January 6 was chaotic, disorganised and more typical of a spontaneous riot.

But Representative Mikie Sherrill said Wednesday that groups of Trump supporters had toured the Capitol for "reconnaissance" a day before the attack, calling the visits "suspicious.

The visitors "could only have gained access to the Capitol Complex from a Member of Congress or a member of their staff," the former Navy helicopter pilot and other lawmakers said in a letter to Capitol security.

"The ties between these groups inside the Capitol Complex and the attacks on the Capitol need to be investigated."

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Much focus is on one video. In it, several protestors inside a Capitol room are regrouping to figure out where to go after their initial success entering the building.

The woman with the bullhorn shouts from just outside through a broken window.

"Hey guys, I've been in the other room," she says.

"In the other room on the other side of this door right here where you are standing, there is a glass that somebody, if it's broken, you can drop down into a room underneath it."

"There's also two doors in the other room. One in the rear, and one to the right as you go in."

"So people should probably coordinate together if you're going to take this building."

Yet, that doesn't add up to a sweeping conspiracy or cogent plan.

Matthew Feldman of the Center for Analysis of the Radical Right in Britain said that's typical of political violence.

At the moment, with limited evidence on communications and other links, he said, "It is impossible to establish causality."

"Those people who made up the herd did not appear organised, but within them clearly there were people who were organised."

At the same time, he noted that the crowd included members of the violent Three Percenters, Oath Keepers and Proud Boys, some who traveled to Washington from as far as Hawaii.

Their presence, the discovery of pipe bombs planted nearby, and the apparent threats to capture members of Congress or the vice president, were signs of "terror" plotting, he said.

"You had a riotous mob... and within that you had domestic terrorists" who were "clearly planning something."

Pelosi on Friday added fuel to the idea that there was planning and coordination, without saying how far it could go.

"If, in fact, it is found that members of Congress were accomplices to this insurrection, if they aided and abetted the crime, there may have to be actions taken beyond the Congress in terms of prosecution for that," she said.