(Reuters) - Twitter Inc TWTR.N 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.

The Twitter logo is shown at its corporate headquarters in San Francisco, California April 28, 2015. REUTERS/Robert Galbraith

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.

Lawyers specializing in terrorism said Fields faces an uphill battle, though the case could lead to more calls for social media companies such as Twitter and Facebook Inc FB.O to take down posts associated with terrorist groups.

In her complaint filed on Wednesday, Fields said San Francisco-based Twitter 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, which was filed in the federal court in Oakland, California.

Fields wants Twitter to pay her triple damages for violating the federal Anti-Terrorism Act by having provided 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 law.

“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 VALLEY

Last 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 also met with technology executives in Silicon Valley last week to discuss what more could be done to counter Islamist militants.

“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 U.S. 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.

David Greene, civil liberties director of the Electronic Frontier Foundation digital rights group, said the Fields case plows new legal ground under the anti-terrorism law.

“With this, the claim is not that you are reaching out or doing something special” for an entity identified as a terrorist group, he said. “This is that they (Twitter) need to stop providing (Islamic State) with the same service they provide the rest of the world.”

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.

Fields said Twitter aided Islamic State “knowingly or with willful blindness,” citing the company’s alleged resistance to requests from Congress, Democratic presidential candidate Hillary Clinton and others to do more to keep the group offline.

TAKEDOWNS

Islamic 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 censor.

According to its online “transparency report,” Twitter honored none of the 25 requests from U.S. government and law enforcement authorities to remove posts between January and June 2015.

Twitter said it honored 42 percent of the 1,003 removal requests from governments, law enforcement and courts worldwide during that period, and withheld 158 accounts and 2,354 tweets. It said more than two-thirds of the requests came from Turkey.

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 ARBK.AM liable for handling transactions for Palestinian militant group Hamas, said there is "no question" the Anti-Terrorism Act covers Fields' case, but showing "knowledge or willful blindness" may be tough.

Arab Bank settled its case in August.

The case is Fields v. Twitter Inc, U.S. District Court, Northern District of California, No. 16-00213.