Two days after a helicopter collided with a passenger jet in Washington in January, killing 67 people, Jo Ellis woke up to a flurry of text messages.

Ms. Ellis, a 35-year-old helicopter pilot in the Virginia Army National Guard, learned from friends that her name and photos were all over social media. Users were falsely naming her as the pilot who had crashed into a passenger jet on Jan. 29 — a sign, in the eyes of the online mob, that diversity initiatives had played a role in the crash because Ms. Ellis is transgender.

She posted a “proof of life” video on Facebook — emphasizing that she was very much alive and well in an attempt to slow the spread, but claims seemed to multiply.

“My life was turned upside-down at that point,” Ms. Ellis said in an interview, adding that her employer sent armed bodyguards to protect her family and she started carrying a loaded weapon as a precaution. “Forever on, I’m known as ‘that trans terrorist.’”

Ms. Ellis filed a defamation lawsuit on Wednesday against Matt Wallace, an influencer on X with more than two million followers. Mr. Wallace was one of the more prominent people to spread the falsehood in a series of posts that included photos of Ms. Ellis and details about her life.

Mr. Wallace deleted his posts about Ms. Ellis after her Facebook video started spreading online. He posted an “important update” on the afternoon of Jan. 31, writing that Ms. Ellis “was not piloting the helicopter that crashed in to the plane and is still alive.”

The filing claims that Mr. Wallace had “concocted a destructive and irresponsible defamation campaign.” It was filed in U.S. District Court in Colorado, the state where Ms. Ellis’s lawyers said Mr. Wallace resides, and seeks monetary damages to be determined at trial.

Mr. Wallace did not immediately respond to requests for comment.

It is difficult for anyone targeted by digital misinformation to find recourse after lies spread about them online. Social media companies have softened their stance on content moderation in recent years, just as misinformation peddlers have become more prominent and closer to centers of power.

At the same time, the idea that social media influencers could be held personally and financially liable through defamation law for spreading overtly false statements online has grown as one potential avenue for combating misinformation.

“This suit situates itself within a clear growing trend,” said RonNell Andersen Jones, a professor of law at the University of Utah who focuses on defamation. “This is all a relatively new and complicated use of defamation law: People victimized by viral conspiracy theories are increasingly attempting to use defamation law not just to remedy their own reputations but to correct the wider societal lie.”

The approach was bolstered in recent years by successful defamation cases against much larger groups: In 2023, Dominion Voting Systems won a $787.5 million settlement against Fox News for spreading lies about its voting machines after the 2020 election. Families tied to the Sandy Hook school massacre sued Alex Jones, the fabulist behind Infowars, for defamation and won more than $1 billion in damages in 2022.

There are fewer examples of such lawsuits against independent creators or social media influencers.

Ms. Ellis’s lawsuit was filed by the Equality Legal Action Fund, a group of mostly volunteer lawyers who defend L.G.B.T.Q. people against defamation and harassment.

Such lawsuits face a number of constitutional and legal hurdles. Free speech laws are broad, making it difficult to prove defamation even when a falsehood is shared. In most cases it’s up to the people who are defamed to prove that the speaker acted with deliberate malice instead of making a mistake.

Ms. Ellis said that any financial compensation she may receive would be donated to the families of the victims in the crash.

“I believe in free speech, but I also believe in consequences to free speech,” Ms. Ellis said. “If you can stir up a mob because you say something that’s not true, that’s your right. But once the mob comes after someone, you’ve got to have some consequences.”

Speculation that a transgender pilot could have caused the collision on Jan. 29 emerged as a conspiracy theory almost immediately after a Black Hawk helicopter on a training exercise collided with a passenger jet over the Potomac River. Just days earlier, President Trump had signed an executive order attempting to bar transgender people from the military, prompting some users to speculate that the crash was an act of terrorism by an aggrieved transgender pilot. Mr. Trump continued to connect the crash to policies related to diversity, equity and inclusion, or D.E.I., for days afterward.

Mr. Wallace was not the first person to target Ms. Ellis on X, according to a review of posts by The New York Times. The conversation around Ms. Ellis began on Jan. 30 and exploded through Jan. 31, becoming a trending topic on X with more than 90,000 posts by the second day, according to Trends24, a website that monitors social media.

“I’ve been a door gunner in a helicopter in Iraq during a combat zone, and I’ve been shot at in that same combat zone,” Ms. Ellis said. “But even for me, having a magnifying glass placed on my personal life in the wake of that rumor had a real impact.”