Mourners gather during a vigil at the White House on Sunday after the fatal shooting in Orlando. (Photo by Oliver Contreras for The Washington Post)

ORLANDO: THE ISIS CONNECTION. The perpetrator of the deadliest mass shooting in American history pledged loyalty to the Islamic State in a 911 call during his attack on a gay nightclub in Orlando. But this isn’t a foreign fighter situation, or one in which a person showed obvious signs of radicalization. Omar Mateen was a native-born American citizen, and although his ex-wife says he was violent and abusive, and his Afghan father hosted an online show that praised the Taliban, both said Mateen showed no signs of religious radicalization. One friend said that Mateen was becoming more religious after his divorce, and that he went on a pilgrimage to Saudi Arabia, but that he never referred to ISIS or other terrorist groups. The details of Mateen’s alleged ties, or at least sympathy, to international terrorist groups remain unclear. But they are adding layers of mystery, complication and fear to a tragedy that has gripped the nation, reawakening debates about gay rights and gun control, as well.

ORLANDO: WHAT THE FBI KNEW. The shooter had twice come under investigation by the FBI for comments suggesting support of Islamic groups and for vague connections to a person who traveled to Syria to become a suicide bomber. But both times, the investigations cleared him. The FBI’s focus on Mateen has raised questions about the stringency of the bureau’s screening process – and whether it can catch a would-be domestic terrorist bent on committing a massive hate crime before he or she carries out an attack.

ORLANDO: THE WARLIKE WEAPON OF CHOICE. The AR-15, an American-made assault-style weapon that the Orlando gunman used to carry out the mass shooting, has a history on the battlefield and in the homeland.

You can find military variants of AR-15s on both sides of the battle against terrorism, in the hands of U.S. allies and of extremist fighters that make up the Islamic State and al-Qaeda. They are standard-issue for U.S. service members. And ever since the federal assault weapons ban expired in 2004, they’ve become popular on the civilian market, available for $800 to $1,800. They even come in pink.

The Orlando tragedy has  fanned the flames of a long-simmering gun control debate in the country. The issue has broken down into pro- and anti-gun-control camps citing competing facts. According to data collected by Mother Jones, most mass shooters get their weapons legally.

Yet assault-style weapons are becoming the go-to gun of choice for mass shooters.

Congress is only too likely to revisit this debate in the wake of the Orlando massacre. But if history is any guide, the discussion may not be resolved. Despite presidential entreaties and a growing list of carnage, the upshot of sharp divisions between lawmakers has been that no meaningful action on gun control, or proposed GOP alternatives focusing more on mental health, has been possible for years.