Media playback is unsupported on your device Media caption Johnny Depp: "When was the last time an actor assassinated a president?"

Actor Johnny Depp has caused controversy after he appeared to threaten US President Donald Trump at the Glastonbury Festival. "When was the last time an actor assassinated a president?" he asked the crowd.

It is a crime in the US to make threats against the president, so could his remarks get him into trouble?

What did he say?

As he introduced a screening of his film The Libertine, Johnny Depp asked the audience: "Can you bring Trump here?"

After receiving jeers from the crowd, he added: "You misunderstand completely. When was the last time an actor assassinated a president?"

The comment seemed to be a reference to the assassination of President Abraham Lincoln by actor John Wilkes Booth in 1865.

"I want to clarify," he added. "I'm not an actor. I lie for a living [but] it's been a while. Maybe it's about time."

Could he be in trouble?

Image copyright Reuters

Under the United States Code (Title 18, Section 871), threatening the US president is a class E felony. Anyone who "knowingly and wilfully" makes "any threat to take the life of, to kidnap, or to inflict bodily harm" upon the US president could be sentenced to up five years and/or be fined. It also includes the vice-president and presidential candidates.

The US Secret Service is the agency tasked with investigating suspected cases. And US media reported that the service was aware of the actor's comments.

But, when it comes to statements made by artists, courts have usually declared the cases to be protected speech under the First Amendment to the US Constitution.

Stanford University Law Professor Nathaniel Persily told USA Today newspaper: "People are allowed to wish the president dead," up to the point they express a real intent to harm him. "To threaten someone you need words that encourage some sort of action."

It's not the first time, is it?

No. Singer Madonna said earlier this year that she had thought "an awful lot about blowing up the White House", sparking furious reaction from Trump supporters. She later said her remarks were taken out of context.

Reacting to the controversy, one Secret Service official told the New York Post that any action against her would depend on whether the remarks were considered a genuine threat - which could also be the case with Johnny Depp now.

The official said then: "It's all about intent. Is she intending to do harm to the White House or President Trump? Otherwise it will be characterised as inappropriate."

Other cases included rapper Snoop Dogg, who shot a toy gun at a Donald Trump character in a music video and comedian Kathy Griffin, who posted a photo in which she appeared to hold a fake bloodied head that resembled Mr Trump.

Media playback is unsupported on your device Media caption Fired comedian Kathy Griffin weeps: "Trump broke me"

And earlier this month, a show in New York depicted the assassination of a Julius Caesar that resembled Mr Trump, sparking not only criticism but also threats.

Despite the angry responses sparked by all those cases, there was no public acknowledgment that officials were investigating them as genuine threats against the president.

But, after the storm caused by Griffin's photos, the Secret Service said on Twitter that it had a "robust protective intelligence division" monitoring "open source reporting and social media" to evaluate threats, adding that:

Image copyright Twitter - SecretService

And here is an example of that: a man in the state of Illinois was charged this month with threatening the president after posts he made on Facebook.

Why so much controversy?

Image copyright Reuters

Perhaps it all goes down to the controversial nature of Mr Trump.

Artists say it is part of their work to be provocative and have often cited freedom of expression to protect what they do and to convey political messages.

But critics, including Trump supporters and the president himself, say many have crossed the line of what it is acceptable, suggesting a double standard when it comes to remarks against the president.

Image copyright Twitter - RealDonaldTrump

But during the presidential campaign last year, Mr Trump himself was accused by his Democratic rival, Hillary Clinton, of inciting violence by saying that gun rights supporters could stop her from winning. He was not investigated.