US Democratic senators are conducting a filibuster in an attempt to force a vote on gun control legislation following the shooting at an Orlando nightclub that left 49 people dead.
Senator Chris Murphy vowed to stay on the Senate floor “until we get some signal… that we can come together”.
Meanwhile, the first wakes have been held for victims of the shooting.
Mourners gathered to remember those killed when gunman Omar Mateen burst into the Pulse gay nightclub on Sunday.
It was the worst mass shooting in modern US history. Dozens of people remain in hospital, some in a critical condition.
In another development, Mateen reportedly made a series of Facebook posts before and during his attack in which he raged against the “filthy ways of the west” and blamed the US for the deaths of “innocent women and children”, a Senate committee letter has revealed.
- How attacks unfolded
- Who was Omar Mateen?
- How do you get denied a gun in Florida?
- What is the NRA and why is it so powerful?
- Orlando shooting: Special report
According to the letter Mateen also said on Facebook: “America and Russia stop bombing the Islamic state.”
The Senate Homeland Security Committee has asked Facebook to provide information on Mateen’s online activity.
Senator Murphy began the filibuster – prolonged speaking on the floor to interrupt other business – at 11:21 and he was still standing several hours later, although addressing a largely empty chamber.
He is from Connecticut, where 26 people died in a shooting at Sandy Hook Elementary School in 2012.
Senator Murphy says he wants to force Republicans and Democrats to agree on legislation to deny suspected terrorists the right to buy guns and require universal background checks.
“For those of us that represent Connecticut, the failure of this body to do anything, anything at all in the face of that continued slaughter isn’t just painful to us, it’s unconscionable,” he said.
Gun control is a divisive topic in the US, where the right to bear arms is enshrined in the constitution.
Earlier on Wednesday, presumptive Republican presidential nominee Donald Trump said anyone on a terror watch list should be prevented from buying guns.
He tweeted that he would meet powerful lobby group the National Rifle Association to discuss the gun control issue.
The NRA responded by saying it would meet him but it already opposes terrorists buying guns.
Until now, Mr Trump has been a strong supporter of protecting gun rights and his candidacy was endorsed by the NRA.
US Vice-President Joe Biden, speaking at a gun-control fundraising event in Washington, said the idea that a suspect on a terror watch list could still legally buy guns was absurd.
He said it had taken seven years for Congress to approve a ban on assault weapons which expired in 2004.
“I refuse to give up. We refuse to give up. It took me seven years to get the first ban put in place. There is no reason why we should ever stop,” he said.
Mr Biden and President Barack Obama are due to visit Orlando on Thursday.
The FBI has two terror “watch lists”. The smaller one bans flying to and from the US and there is also a larger one, which Mateen was on.
Mateen was put on that list for 10 months while under investigation following inflammatory comments at work.
But the FBI concluded there was no evidence he was a terror threat.
The 29-year-old, a US national with Afghan parents, bought an assault rifle and a handgun in early June.
Mateen pledged allegiance to so-called Islamic State as he carried out the attack and people who knew him said he displayed an apparent hatred towards gay people.
His wife is also being questioned in connection with the atrocity.