SAN FRANCISCO — In the hours after 14 people were killed in San Bernardino, California, a familiar voice celebrated the attacks on Twitter: “California, we have already arrived with our soldiers. Decide how to be your end, with knife or bomb.”

That comment was posted from the 335th Twitter account of a pro-Islamic State group that calls itself Asawitiri Media.

Twitter has repeatedly tried to cut off the authors of the account, most recently known to its thousands of followers as @TurMedia335, @TurMedia334 and @TurMedia333.

As soon as Twitter suspends one account, a new one is created.

After the group’s 99th account was suspended, it taunted Twitter by creating @IslamicState100, posting images of birthday candles, cake, trophies and fireworks.

Politicians and even some technologists say that account, and hundreds just like it, show how Silicon Valley’s efforts to crack down on the use by terrorists of social media and other technologies, like encryption, have been toothless. And on Sunday, Hillary Clinton, the Democratic presidential front-runner, called on tech companies to become more aggressive.

“Resolve means depriving jihadists of virtual territory, just as we work to deprive them of actual territory,” she told an audience at the Brookings Institution in Washington. “They are using websites, social media, chat rooms and other platforms to celebrate beheadings, recruit future terrorists and call for attacks. We should work with host companies to shut them down.”

But as the 335 versions of the pro-Islamic State Twitter account demonstrate, technology companies are dealing with a tenacious adversary. Also, when it comes to terrorists using encryption technologies to hide their communications — a frequent complaint of law enforcement — technology companies are quick to point out of the top five encryption apps recommended by the Islamic State, none are American-made.

Indeed, members of the Islamic State used technology created by a company in Germany to claim credit for last month’s attacks in Paris, and text messages discovered on one of the attackers’ phones suggested the attackers were not even using encryption.

A tall order

Still, while the tech industry cannot block all terror content on the Web, people who have battled other online threats say it could be doing more. They point to technology that has successfully eradicated large swaths of child pornography on the Web, or even older tools for spotting computer viruses, as potential guideposts for blocking terror-related content.

“When Twitter says, ‘We can’t do this,’ I don’t believe that,” said Hany Farid, chairman of the computer science department at Dartmouth College, who co-developed the child pornography tracking system with Microsoft. The actual task of identifying child pornography is managed by the National Center for Missing & Exploited Children.

Farid said the same technology could be applied to terror content, so long as companies were motivated to do so. “There’s no fundamental technology or engineering limitation,” he said. “This is a business or policy decision. Unless the companies have decided that they just can’t be bothered.”

In response, Twitter said it actively investigated potential terrorist threats. “Violent threats and the promotion of terrorism deserve no place on Twitter, and our rules make that clear,” a Twitter spokesman said. “We have teams around the world actively investigating reports of rule violations, and they work with law enforcement entities around the world when appropriate.”

A complex issue

Conversations between government officials and tech companies have been going on for more than a year, but since the mass shootings in San Bernardino and Paris, “there has been a reintensified or reinvigorated engagement,” according to a senior administration official who spoke on the condition of anonymity.

But some who advocate free speech argue that relying on companies like Twitter and Facebook to be more aggressive arbiters of what constitutes free speech puts those companies in a difficult spot.

“We don’t believe that law enforcement should delegate their responsibilities to private enterprise,” said David Greene, director for civil liberties at the Electronic Frontier Foundation. “Especially ones that haven’t sought out that role.”

Pulling all terror-related content is not always preferred by law enforcement. In several cases, tech executives say, they have been asked to keep terror-related content online so law enforcement agents can monitor terrorist networks or because the content was created by law enforcement agents to lure terrorists into divulging information.

The issue is thornier for companies like Facebook, in which the bulk of posts are meant to be private. “Do you want Facebook looking at over 1.5 billion people’s posts?” said Zeynep Tufekci, an assistant professor in technology policy at the University of North Carolina at Chapel Hill. “And if so, then for what?”

Facebook primarily relies on user reports to ferret out terrorist accounts, but recently it has gone further. If the company is informed of specific terrorist activity, Facebook will take down the account as well as others similar to the one reported.

Last week, it went a step further. Tashfeen Malik, one of two terrorists involved in the San Bernardino attacks, posted her public allegiance to ISIS on Facebook just minutes before the shootings Wednesday. The post did not trigger any user reports to Facebook, and until now it had been unclear how the authorities were able to tie Malik to the post.

According to law enforcement officials, the Facebook account Malik used was linked to an email account she had used for many years. Facebook found the posting, reported it to the FBI, and removed it Thursday.

“We work aggressively to ensure that we do not have terrorists or terror groups using the site, and we also remove any content that praises or supports terrorism,” said Monika Bickert, head of global product policy at Facebook.