The National Rifle Association (NRA) is suing Florida after it passed a gun control law in the wake of a school shooting that left 17 people dead.
Governor Rick Scott, a staunch ally of the gun lobby, enacted the bill, which the NRA says violates the constitution.
The law raises the legal age for buying rifles in Florida, but also allows the training and arming of school staff.
It does not ban semi-automatic rifles like the one used in the 14 February massacre in Parkland.
But it does introduce a three-day waiting period on all gun sales and a ban on bump stocks, a device that enables semi-automatic rifles to fire hundreds of rounds a minute.
The NRA filed its lawsuit on Friday just an hour after the bill was signed by the governor.
One of its arguments is that the legislation violates the rights of young women as they are unlikely to commit violent crime.
What's in the new law?
- It raises the minimum age for buying rifles from 18 to 21 in the state - although 18, 19 and 20-year-old police officers and members of the security forces will still be able to buy rifles and shotguns
- It bans bump stocks - devices that raise the firing speed of semi-automatic rifles
- It introduces a three-day waiting period on all gun purchases (previously this only applied when people bought handguns)
- It makes it easier for police to confiscate weapons and ammunition from people who are deemed to pose a threat of violent behaviour (a measure that has been proposed by five other states in the last month, according to the Brady Campaign to Prevent Gun Violence)
- It allows school staff to carry guns, with the agreement of their school district authorities and sheriff's department. This is already allowed in the states of Wyoming, South Dakota, Tennessee, Georgia, Kansas and Texas
President Donald Trump has voiced support for arming teachers and so has the NRA.
Florida Governor Rick Scott, a Republican, signed the bill into law on Friday, after lawmakers at the Florida state level voted in its favour. As he signed it, he said he was an NRA member, and that some members would agree with the new law while others would not.
"It's an example to the entire country that government can and has, moved fast," he said.
Florida's Attorney General Pam Bondi said the changes were "the right thing to do".
"This bill is not perfect, and sadly it will not bring back the 17 lives lost in the horrific school shooting, but the safety of our children is not a political issue," she said.
What does the NRA say in its lawsuit?
The NRA is asking a judge to block the age restriction and prevent the state of Florida from enforcing the new law.
People aged 18 are considered adults "for almost all purposes and certainly for the purposes of the exercise of fundamental constitutional rights", the lawsuit says.
The complaint says the law violates the second amendment of the US constitution, which protects the right to bear arms.
It also argues the bill breaches the 14th amendment's equal protection clause by banning law-abiding citizens between 18-21 from buying guns.
NRA spokesman Chris Cox said the bill "punishes law-abiding gun owners for the criminal acts of a deranged individual".
"Preventing a responsible 20-year-old from purchasing the best tool for self-defence will not stop a deranged criminal intent on committing a crime," the lobby group said.
The legal action says the Florida legislation particularly affects young women.
"Females between the ages of 18 and 21 pose a relatively slight risk of perpetrating a school shooting... or, for that matter, a violent crime of any kind," says the lawsuit.
It also points out that men of 21-24, just outside the new age limit, account for more violent crime arrests than men aged 18-21.
How did we get here?
A former student with a history of mental health issues is accused of killing 14 students and three staff members at Marjory Stoneman Douglas High School on 14 February in the second-deadliest US school shooting ever.
The bill's passage by a Republican-controlled legislature in a state where the NRA wields considerable influence is seen as a testament to an impassioned pro-gun control campaign launched by young survivors of the shooting and parents of the victims.
The bill was signed into law in the presence of some Parkland survivors and their families, while some of the student campaigners from the school welcomed it as a good first step.
What reaction has there been?
"How am I not surprised" at the NRA's lawsuit, wrote David Hogg, one of the students from the Parkland school who have turned to campaigning since the shooting.
In California, Democrat state senator Scott Wiener said the NRA's arguments were "astounding" and called the group a "toxic force".
The #NRA - perhaps the most toxic force in U.S. politics - just sued Florida based on the astounding argument that 18 year olds have a constitutional right to buy assault rifles. The NRA is to public safety what climate deniers are to the environment. Time for the NRA to go away.— Scott Wiener (@Scott_Wiener) March 9, 2018
But there were plenty of others who were opposed to the new Florida law.
WTH.— Heather (@hrenee80) March 10, 2018
If a person wants to shoot someone, it is highly doubtful that they are going to just wait patiently until they turn 21 to be “legal” to buy a gun, and then go shoot people
It is definitely “illegal” to shoot & kill innocent people.
We see how well that works out#NRA