Kathy Griffin sparked outrage across the political spectrum Tuesday when a photo surfaced of the edgy comedian holding aloft the bloody head of a dummy made to look like President Trump.

Much of the outcry on social media called for the Secret Service, the agency tasked with looking out for the president's safety, to arrest Griffin for threatening the president.

Griffin herself said she "crossed the line" in a video apology. But no matter how guilty Griffin was of bad taste, did she commit a crime?

Threats "knowingly and willfully" made against the president, president-elect, vice president or vice president-elect are a class E felony under federal law. That includes any "threat to take the life of, to kidnap, or to inflict bodily harm" upon those officials.

But, in Griffin's case, there is no question the photo of her holding a mock Trump head is protected speech, said Stanford University Law Professor Nathaniel Persily.

The photograph did not directly threaten the president and it didn't urge other people to harm him, Persily said.

"People are allowed to wish the president dead," up to the point they express a real intent to harm him, Persily said. "To threaten someone you need words that encourage some sort of action," and those words are absent in Griffin's photo, he said.

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In a 1969 ruling, the Supreme Court overturned the conviction of Robert Watts, a young man accused of threatening former president Lyndon Johnson. Watts said at a 1966 political rally that he was "not going" if he was drafted to serve in Vietnam and added that "if they ever make me carry a rifle, the first man I want to get in my sights is L.B.J."

The Supreme Court ruled Watts' statement, which is much closer to meeting the definition of a threat than Griffin's photos, was merely "crude political hyperbole."

"In light of its context and conditional nature," Watts speech "did not constitute a knowing and willful threat against the President," the court ruled, citing the importance of the First Amendment.

"A statute such as this one, which makes criminal a form of pure speech, must be interpreted with the commands of the First Amendment clearly in mind," said the court. "What is a threat must be distinguished from what is constitutionally protected speech."

Determining which is which is all about context and the intent of the speaker, Persily said. "You have to get into a person's head" to determine if they really wanted to harm the president and weren't being satirical or making a crude political point.

While the Secret Service did not mention Griffin by name, the agency appeared to address the controversy in two tweets posted after the images had gone viral on social media.

Persily cautioned against any attempt to prosecute Griffin because efforts to stifle speech usually just bring more fame to the speaker and more attention to their grievances, he said.

"When you try to clamp down on offensive speech, you actually expose more people to it," Persily said.