Metro buses and trains, such as this one at the Arlington Cemetery station on April 14, 2015, won’t be displaying any issue-oriented ads for the rest of the year. (Matt McClain/The Washington Post)

Metro said Thursday that it will not allow new issue-oriented advertising in the transit system after a controversial pro-Israel group sought to place ads featuring a cartoon of the prophet Muhammad, a drawing that was linked to deadly violence in Texas this month.

The ban will extend through at least the end of the year.

The decision by Metro’s governing board came as a surprise, given that the topic was not on its Thursday meeting agenda. In the terrorism age, the board’s sudden unanimous vote reflects growing concern among the nation’s biggest transit systems over anti-Muslim-themed ads paid for by the American Freedom Defense Initiative, or AFDI.

The group — which has repeatedly tangled in court with transit agencies, including Metro, over provocative advertising — sponsored the recent Muhammad Art Exhibit and Contest in Garland, Tex., in which a $10,000 prize was awarded for the best caricature of the prophet Muhammad. Many Muslims consider such depictions to be blasphemous.

The contest ended in bloodshed May 3 after two men, Nadir Hamid Soofi and Elton Simpson, opened fire with semiautomatic rifles outside the building in the Dallas suburb where the contest was held. Both were shot and killed by a police officer. A security guard suffered a leg wound, but no one else was injured.

Metro said AFDI wanted to feature the winning cartoon in advertisements in five subway stations and on 20 buses throughout June, at a cost of about $20,000. Rather than prohibit those ads specifically — a move that federal courts have forbidden — Metro opted to ban all new advocacy ads for the rest of the year.

Less controversial advocacy ads include political campaign messages, ads related to environmental issues and messages about government spending or legislative debates. All are common in the transit system serving Washington.

“My view is, you put that ad up on the side of a bus, you turn that bus into a terrorism target,” a top Metro official said Thursday, speaking on the condition of anonymity because the agency anticipates being sued over the ban. “I think it’s a very bad outcome for everybody. But it’s a risk we don’t want to put our passengers under.”

AFDI’s president and co-founder, Pamela Geller, reacted strongly to Metro’s decision.

“These cowards may claim that they are making people safer, but I submit to you the opposite,” she said in an e-mail. “They are making it far more dangerous for Americans everywhere. Rewarding terror with submission is defeat. Absolute and complete defeat.”

Geller, an uncompromising advocate for a strong Israel whose views on the Muslim world have been called Islamophobic by critics, referred to Islamic law in describing the Metro ad ban. “This is sharia in America,” she declared.

She did not say whether she intends to take legal action.

Transit officials said the board, after more consideration, could vote to make the ban on issue-related ads permanent or extend it into 2016, a presidential election year, when demand for campaign ad space would be exceptionally high.

After New York’s Metropolitan Transportation Authority tried to selectively ban AFDI ads from that city’s transit system, the group won a federal court challenge of the ban last month. The MTA responded by prohibiting all issue-related ads.

Like Metro, the Southeastern Pennsylvania Transportation Authority, which runs Philadelphia’s subway and bus systems, decided Thursday to ban all advocacy messages after a judge prohibited SEPTA from specifically rejecting AFDI ads. The Chicago Transit Authority has banned advocacy ads since 2013.

“I think there’s a potential threat and a danger if we were to accept that ad,” Metro board member Michael Goldman said. “Better to be safe than sorry. I mean, this is the nation’s capital. If anything is going to happen, it’s probably going to happen here.”

Lynn Bowersox, Metro’s marketing manager, said revenue from issue-oriented ads accounts for about 20 percent, or roughly $2.5 million, of the agency’s overall annual income from advertising. Advocacy ads, if they had been allowed to continue, would have brought in about $1 million in the remaining seven months of this year, she said.

She said new commercial advertisements could offset some of that $1 million from now to December. Issue-related ads currently in the transit system will be allowed to stay until the contracts for them expire, Bowersox said.

David Marburger, a Cleveland lawyer specializing in First Amendment issues, said Metro and other transit agencies appear to be on solid legal ground in banning all advocacy ads, based on a 1974 U.S. Supreme Court case, Lehman v. City of Shaker Heights.

A candidate for the Ohio House of Representatives who wanted to buy campaign ad space in the Shaker Heights transit system filed a lawsuit challenging the system’s ban on issue-oriented advertising. He argued essentially, that, prohibiting free speech on a public bus or commuter train was legally the same as banning it on a street corner or in a public park.

“And the court’s answer was no,” Marburger said. The majority ruling in the 5-to-4 decision held “that the advertising spaces on a city’s transit system are not traditional First Amendment forums,” he said. Moreover, “when the government operates a transit system, there’s an underlying suggestion that the government is endorsing the message of the public speech. And the government doesn’t want to look like it is.”

Agencies get into trouble, Marburger said, when they try to ban only certain advocacy ads, as Metro and others have discovered.

In September 2012, AFDI bought ad space in four Metro stations for the message, “In Any War Between the Civilized Man and the Savage, Support the Civilized Man. Support Israel. Defeat Jihad.” Metro wanted to delay displaying the ads because of unrest in the Middle East, but AFDI sued and a judge ordered the ads to be posted.

When AFDI bought space on Metro buses last year for ads linking Islam to Nazism, the transit agency quietly accepted the advertisements. This spring, AFDI sought to place the same ad in the Philadelphia transit system. SEPTA said no. AFDI successfully sued, prompting the system to ban all advocacy ads.