Twitter, Facebook and Google 'aided Paris attacks'
Tech companies are facing legal action from the father of a woman killed in the Paris attacks last November.
Reynaldo Gonzalez is accusing Google, Facebook and Twitter of providing "material support" to extremists.
He alleges they "knowingly permitted" the so-called Islamic State group - referred to as "ISIS" in his legal action - to recruit, raise money and spread "extremist propaganda".
The companies said they had policies against extremist material.
Mr Gonzalez's daughter Nohemi was among the 130 people killed when extremists attacked Paris' Bataclan concert hall, bars, restaurants and the national football stadium in nearby Saint-Denis.
"For years, [the companies] have knowingly permitted the terrorist group ISIS to use their social networks as a tool for spreading extremist propaganda, raising funds and attracting new recruits," court papers filed in California on Tuesday read.
"This material support has been instrumental to the rise of ISIS, and has enabled it to carry out numerous terrorist attacks, including the 13 November 2015 attacks in Paris, where more than 125 were killed, including Nohemi Gonzalez."
Mr Gonzalez alleged that, without Twitter, Facebook, and the Google-owned YouTube, the "explosive growth of ISIS over the last few years into the most-feared terrorist group in the world would not have been possible" because they had provided the infrastructure necessary for the group to get its message out.
He quoted Brookings Institution research as saying Islamic State had "exploited social media, most notoriously Twitter, to send its propaganda and messaging out to the world and to draw in people vulnerable to radicalisation".
In statements given to the Associated Press news agency, Facebook and Twitter said the case was without merit, and all three companies cited their policies against extremist material.
Twitter, for example, said it had "teams around the world actively investigating reports of rule violations, identifying violating conduct, and working with law-enforcement entities when appropriate".
Facebook's statement said if the company saw "evidence of a threat of imminent harm or a terror attack", it would contact law enforcement.
Google said it would not comment on pending legal action, but noted that it had "clear policies prohibiting terrorist recruitment and content intending to incite violence and quickly removed videos violating these policies when flagged by our users".
Under US law, internet companies are generally exempt from liability for the material users post on their networks.
Section 230 of the 1996 Communications Decency Act provides a legal "safe harbour" for companies such as Twitter and Facebook, stating that "no provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider".
But it is unclear whether that legal defence would suffice in this case.
Ari Kresch, one of the lawyers representing Mr Gonzalez, told AP: "This complaint is not about what ISIS's messages say… it is about Google, Twitter, and Facebook allowing ISIS to use their social media networks for recruitment and operations."
The legal action also alleges that YouTube shared revenue with the terror group from adverts that ran with its videos.
Benjamin Wittes, a senior fellow at the Brookings Institution, agreed that the legal "safe harbour" might not shelter social-media companies in such cases.
But he told AP Twitter the causal link between the alleged support for extremists and the attack was very weak.