A Fremont man accused of planning to join an al Qaeda affiliate in Syria is being charged under a sweeping federal law that has been used to prosecute those who provide a terrorist organization with training and supplies, make charitable contributions, or even offer advice on how to work peacefully within the political system.

Justice Department filings unsealed Thursday revealed that Adam Shafi, 22, had been jailed since July 3, four days after federal agents stopped him from boarding a flight from San Francisco to Turkey. From there, prosecutors say, Shafi planned to go to Syria and fight alongside the al-Nusra Front, an offshoot of al Qaeda that is trying to overthrow Syrian President Bashar al-Assad and was designated a terrorist organization by the United States in December 2012.

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“If you’re aware that it’s an organization that does terroristic things and you give resources that could be diverted to the terrorist goal … you could be guilty,” said Robert Weisberg, a Stanford criminal law professor. He called the law “unbelievably broad.”

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The court has been protective of free speech in other contexts, such as political contributions and protests at funerals. But in a terrorism case, Weisberg observed, “the court said the First Amendment has some limits.”