(Bloomberg) -- Shares of the biggest video-game makers tumbled on Monday after President Donald Trump blamed the industry for contributing to a culture of violence in the U.S.

“We must stop the glorification of violence in our society,” Trump said at the White House after mass shootings killed 30 people. “This includes the gruesome and grisly video games that are now commonplace.”

Activision Blizzard Inc., Electronic Arts Inc., Take-Two Interactive Software Inc. all fell at least 5% following the remarks, showing investors are worried about a backlash against shooter titles like “Call of Duty” and “Fortnite.” Trump’s remarks weren’t the only headwinds for the stocks. The Nasdaq fell by the most since May over renewed trade tensions with China.

The industry contends that there’s no link between their games and real-life violence. But the companies have frequently been blamed for violent incidents over the years, by both Republicans and Democrats.

“More than 165 million Americans enjoy video games, and billions of people play video games worldwide,” the industry’s trade group, the Entertainment Software Association, said Monday. “Yet other societies, where video games are played as avidly, do not contend with the tragic levels of violence that occur in the U.S.”

Companies adopted a movie-style rating system in 1994 -- under congressional pressure -- that limits sales of the most graphic games to customers over age 17.

“It’s too easy today for troubled youth to surround themselves with a culture that celebrates violence,” Trump said. “We must stop or substantially reduce this, and it has to begin immediately. Cultural change is hard, but each of us can choose to build a culture that celebrates the inherent worth and dignity of every human life.”

Representatives from the industry met with the president in the White House in March of last year, following the school shooting in Florida that left 17 dead.

Research on the subject has been mixed. The American Psychological Association found a link between violent video games and aggression in children, although not necessarily lethal violence. A study published in May in the Journal of the American Medical Association said children who play violent video games were more likely to show dangerous behavior with guns.

Mass murderers from Columbine, Colorado, to Christchurch, New Zealand, have been gamers.

Trump isn’t the only one taking up the issue. Politicians such as House Minority Leader Kevin McCarthy, a California Republican, and Texas Lieutenant Governor Dan Patrick, also a Republican, blamed video games in the wake of the latest shootings.