The huge cache of Isis propaganda videos found on the cellphone of the suspect charged in Tuesday’s terrorist attack in New York has raised questions over the success of efforts to tackle extremist content online.

Sayfullo Saipov, 29, who is accused of using a truck to mow down pedestrians and cyclists on Tuesday, killing eight and injuring 12, told prosecutors he was inspired to carry out the truck attack by Isis videos he had watched on his mobile phone, which was loaded with 90 videos and 3,800 images. Among the files were depictions of beheadings, and shootings, bomb-making instructions and several images of Isis leader Abu Bakr al-Baghdadi.

The source of Saipov’s propaganda is not clear at this point, but we know that Isis has a highly polished propaganda distribution network that feeds well-produced videos, magazines, sermons and photos across the internet. The propaganda machine takes advantage of social media networks like Twitter, Facebook, Google Plus and YouTube, encrypted messaging apps like Telegram and file-sharing platforms like JustPaste.it.

The tech firms have struggled to balance their mission to support free speech with the need to curb the spread of terrorist content. Years of criticism over the way terrorist groups have used their platforms led Facebook, YouTube, Twitter and Microsoft to create a joint forum to counter terrorism in June. The companies have also developed artificial intelligence tools to automatically detect and remove extremist content and launched efforts to provide an alternative narrative to people at risk of being radicalized.

However, some counterterrorism researchers are skeptical about the effectiveness of the alliance and shared many examples of live propaganda accounts on the open web including a Twitter account sharing download links for Isis’s online magazine Rumiyah, a Google Plus account explaining the ideal vehicle and venue for a truck attack and a Facebook account featuring an Isis flag in both the profile picture and cover photo.

“They are catching the top of the iceberg, but not going deep under the iceberg” said Eric Feinberg from cyber intelligence firm GIPEC, who studies the spread of extremist propaganda across platforms online. “I’m sitting here finding this stuff every day and what are they doing? They’re being reactionary.”

Without a coordinated approach between all of these companies, removing terrorist content will continue to be a game of “whack-a-mole”, he added.

Seamus Hughes, the deputy director of the Program on Extremism at George Washington University, said that the propaganda was “not as abundant as it used to be”, but it’s still there and Isis has been very good at evolving its content strategy to evade detection.

He said that Facebook has been the most proactive in tackling the problem, not only taking down content but also launching counterspeech programmes. However, he notes that Google Plus is being used “especially by American jihadis”.

The big technology companies have a “moral and corporate responsibility” to tackle the problem, Hughes said, but ultimately it’s incumbent on governments.

Hughes added that one way technology companies can make a difference is by helping the smaller, less well-resourced companies such as Telegram and JustPaste.it – which don’t have large content moderation teams – to deal with extremist material.

Counterterrorism expert Michael Smith believes Saipov most likely gathered his material from a Telegram channel or Archive.org, the webpage archiving tool he describes as “a clearinghouse for jihadist propaganda”.

Smith, who has infiltrated many Isis Telegram channels and has spent years analyzing the organization’s propaganda, described this week’s attack as “absolutely consistent with everything Islamic State has been petitioning supporters in the US to do since 2014”.

The type of violent content found on Saipov’s phone is part of a strategy to de-sensitize would-be terrorists who don’t have any history of violent crime.

The Islamic State’s last stronghold on the ground might have fallen, but it has been far more successful at building a digital caliphate.

“This latest attack shows that the Islamic State’s power of persuasion has not been diminished,” Smith said.