The US government is struggling to counter the Islamic State group's fast-paced online propaganda, which played a role in inspiring a failed attack in Texas this week on an exhibit of caricatures of the Prophet Mohammed.

One of the two gunmen had been in frequent contact via Twitter with an American militant from the IS group who was well known to federal authorities, according to groups that monitor extremists online.

The failed attempt to storm the cartoon exhibition in a Dallas suburb could be a "harbinger" of things to come, as the jihadist online blitz seeks to encourage violence from a distance, said author Peter Bergen of the New America Foundation non-profit group.

The lightning tempo and vast scale of the IS group's social media campaign poses a daunting challenge, particularly for a government bureaucracy ill-equipped to respond quickly or to experiment, experts said Thursday.

The IS can rely on "a very large number of people" to promote their message online and "it can afford to have 2,000 people who tweet 150 times a day," said J.M. Berger, a fellow at the Brookings Institution.

"It can afford to have a ratio of two or three recruiters to every one potential recruit who might carry out a lone wolf attack," Berger, who has researched extremists' use of social media, told the Senate Homeland Security committee.

The United States or others opposed to the IS are losing the war in social media and will need to deploy similar numbers to have an effect, said Berger, author of "Jihad Joe: Americans Who Go to War in the Name of Islam."

Although the IS jihadists tried to take credit for Sunday's attack, Pentagon chief Ashton Carter said the group had not ordered the assault but had nevertheless "inspired" the men to carry it out.

He said it was "concerning that there are individuals like this who draw their inspiration" from the group.

- Emotional connections -

Social media enabled the IS to find a small but committed group of sympathizers and forge intense connections, Berger said.

"Somebody tweeting from Syria, who's a member of ISIS (IS), can develop a very emotionally powerful relationship with somebody who's sitting in the United States," he said.

Berger and other experts called on US officials to declassify photos, videos, intercepted communications or other intelligence that could expose the failures and excesses of the jihadists in areas in Iraq and Syria under its control.

Daveed Garstein-Ross, a senior fellow at the Foundation for Defense of Democracies, said "the US government has to be able to be more quick to react, to be able to respond at the same kind of speed" as the jihadists online.

In some cases, false claims by the IS have been picked by news media, and the US government needs to promptly release information to show an accurate picture that could undercut the jihadists, he said.

Senator Cory Booker ridiculed the US government's online effort against IS as "laughable."

Holding up an iPad at the hearing, Booker cited a paltry number of retweets for a State Department-backed Twitter account, "Think Again Turn Away," which is supposed to combat IS social media.

Private "hacktivists" -- as well as some foreign governments -- have proven the most effective at combating the jihadists in social media, often by reporting inflammatory posts and getting pro-IS users suspended from Twitter, according to Berger.

As a result, the number of pro-IS Twitter accounts has been significantly reduced in recent months, he said.

- Civil liberties question -

Chasing IS propagandists off Twitter had a downside, though, because it deprived government authorities of information used to track recruitment, he added.

The problem also raised questions about balancing civil liberties with security.

US intelligence and law enforcement agencies have an interest in looking at extremist social media accounts that have been closed or suspended -- or demanding companies keep an archive of such accounts.

"But the appetite in the country probably is not very friendly to the idea that the FBI should be vacuuming up thousands and thousands of social media accounts," Berger said.

Although some plots have been detected on social media, online posts more often provide an "evidence trail" after a suspect is identified, he said.