Twitter Inc is being sued by the widow of an American killed in Jordan who accuses the social media company of giving a voice to Islamic State, adding to the pressure to crack down on online propaganda linked to terrorism.Tamara Fields, a Florida woman whose husband Lloyd died in the Nov. 9 attack on the police training center in Amman, said Twitter knowingly let the militant Islamist group use its network to spread propaganda, raise money and attract recruits.She said the San Francisco-based company had until recently given Islamic State, also known as ISIS, an "unfettered" ability to maintain official Twitter accounts."Without Twitter, the explosive growth of ISIS over the last few years into the most-feared terrorist group in the world would not have been possible," according to the complaint filed on Wednesday in the federal court in Oakland, California.Fields accused Twitter of violating the Anti-Terrorism Act, which allows triple damages for providing material support to terrorists.Her lawyer said he believes it is the first case in which a social media company is accused of violating that federal law.The lawsuit may add to pressure that social media companies such as Twitter and Facebook Inc face to take down posts associated with terrorist groups."While we believe the lawsuit is without merit, we are deeply saddened to hear of this family's terrible loss," Twitter said in a statement about the civil lawsuit. "Violent threats and the promotion of terrorism deserve no place on Twitter and, like other social networks, our rules make that clear."PRESSURE ON SILICON VALLEYLast Friday, the Obama administration set up a task force to crack down on extremist groups using the Internet to advance their goals, find recruits and plan attacks such as recent killings in Paris and San Bernardino, California.Senior national security officials from the administration also met with technology executives in Silicon Valley last week to discuss what more could be done to counter Islamist militants.Fields, the widow, may face an uphill battle to prove Twitter knew or should have known that its technology was helping terrorists."We certainly know social media plays an important role in allowing ISIS to recruit foreign fighters," said Jimmy Gurule, a University of Notre Dame law professor and former US Treasury Department official specializing in terrorist financing."But at the end of the day, is there a sufficient nexus between ISIS' use of Twitter and acts of terror?" he continued. "I'm not saying you can't show it but it's a real challenge."Lloyd "Carl" Fields was among five people killed in the "lone wolf" attack at the police training center by Jordanian police officer Anwar Abu Zeid.The government contractor, who had been a police officer for a decade, was in Jordan to train police from that country, Iraq and the Palestinian territories.Joshua Arisohn, a partner at Bursor & Fisher representing Tamara Fields, said his client can prevail by showing that Twitter's activity was a substantial factor in her late husband's death, and that the death could have been foreseen."Given the significant support that Twitter has knowingly provided to ISIS over the years, we're confident that we can meet this standard," Arisohn said in an email.TAKEDOWNSIslamic State, which controls large areas of Iraq and Syria, has used the Internet regularly to spread its message.The Brookings Institution think tank has estimated that Islamic State supporters operated at least 46,000 Twitter accounts between September and December 2014.Social media companies are not uniform in handling requests from authorities to take down online material. Some technology executives worry that being too quick to remove suspect posts could invite endless and often meritless demands for takedowns.Twitter has positioned itself as a defender of free speech and been reluctant to act as censor.According to its online "transparency report," Twitter honored none of the 25 requests from US government and law enforcement authorities to remove posts between January and June 2015.Worldwide, Twitter said it honored 42 percent of the 1,003 removal requests from governments, law enforcement and courts during that period.More than two-thirds of the requests came from Turkey. Twitter said it withheld 158 accounts and 2,354 tweets during the period.In December, Twitter updated its policies for policing content to explicitly prohibit "hateful conduct."Gary Osen, a lawyer who in 2014 convinced a Brooklyn, New York jury to hold Jordan's Arab Bank Plc liable for handling transactions for Palestinian militant group Hamas, said there is "no question" the Anti-Terrorism Act covers Fields' claims, but that showing Twitter's "knowledge or willful blindness" is the challenge.Fields said she met that standard, citing Twitter's alleged resistance to numerous requests from US. government officials, Democratic presidential candidate Hillary Clinton, and others to do more to keep Islamic State off Twitter.Arab Bank settled its case in August.The case is Fields v. Twitter Inc, US District Court, Northern District of California, No. 16-00213.