Almost exactly a year ago the US gay community was celebrating one of its most poignant victories — the Supreme Court ruling that same-sex marriage was protected by the constitution. Now it is reeling from the most lethal mass shooting in American history.

On Saturday evening the Pulse nightclub touted its Latin theme for the night, posting on Facebook: “Calling all our Latinos, Latinas & everyone that loves a little Latin flavour! It’s time to party tonight! Here’s a little something to play while getting sexy tonight.” But by 1.09am on Sunday morning it posted a chilling update: “Everyone get out of pulse and keep running.”

Authorities were on Sunday evening still struggling to piece together details about the assailant, Omar Mateen, a US citizen of Afghan descent. But the choice of target was clear: a well-known gay nightclub in Orlando, suggesting it was a deliberate and targeted assault on the lesbian, gay, bisexual and transgender community.

That was the conclusion drawn by Marco Rubio, Florida’s Republican senator. “[If] this is something inspired by radical ideology, then I think common sense tells you that he targeted the gay community because of the views that exist in the radical Islamic community about the gay community,” Mr Rubio told CNN on Sunday.

Clubs that cater to LGBT patrons have faced violent attacks in the past, often from the far right. Eric Rudolph, an ultraconservative, in 1997 detonated a nail bomb at the Otherside gay club in Atlanta that wounded five people. More recently, a man named Musab Masmari attempted to burn down a Seattle gay club on New Year’s Eve in 2013. The fire was quickly extinguished and no one was injured. The California native said he was drunk at the time, and was sentenced to 10 years in jail.

While no terrorist group immediately claimed credit for the Pulse assault, an organisation believed to be run by Isis sympathisers called Online Dawah Operations celebrated the attack on Telegram, an encrypted communications phone app. “US crying for 20 homos who got killed. Lol,” one posting said, according to the New York Times. The channel encouraged its supporters to “attack” a Facebook group that was posting updates on the atrocity.

The Orlando massacre called to mind the Isis attack on the Bataclan theatre in Paris last November: an assailant turning high-powered weapons on revellers at a festive — and vulnerable — venue.

In the US, it follows last year’s San Bernardino attack, where a married couple — radicalised by online jihadism and termed “homegrown violent extremists” by Federal Bureau of Investigation director James Comey — murdered 14 people and injured 22 in a mass shooting and attempted bombing. Until this weekend, it was the second-worst terrorist attack on US soil since September 11 2001.

In the wake of San Bernardino, US President Barack Obama warned that the terrorist threat had “evolved into a new phase”.

The latest assault was denounced by American Muslims, with the Council on American-Islamic Relations saying in a statement: “We condemn this monstrous attack and offer our heartfelt condolences to the families and loved ones of all those killed or injured. The Muslim community joins our fellow Americans in repudiating anyone or any group that would claim to justify or excuse such an appalling act of violence.”

Israel, which also boasts a vibrant gay community, has suffered a string of attacks in recent years, too, such as a shooting at the Israeli LGBT Association’s Tel Aviv branch in 2009 which killed two people. Last year an ultraorthodox Israeli stabbed six marchers in Jerusalem’s annual Gay Pride Parade.

The tragedy is a wrenching shift from the sense of progress that the American LGBT community has enjoyed in recent years, culminating in the landmark Supreme Court decision that was joyously celebrated at the annual Gay Pride parades across the US last summer. The victory marked decades of progress towards LGBT rights, with mainstream attitudes across the US shifting decisively in favour of equal rights in recent years.

At the time Steven Hawkins, executive director of Amnesty International USA, noted that “while much work remains to be done to ensure that all forms of discrimination against LGBT people are eliminated once and for all, this long-awaited and significant decision affirms that same-sex couples and their families deserve the same respect and recognition as anyone else”.

But the sense of optimism that has enveloped the community in recent years suffered a horrific setback with this weekend’s attack. In a statement Equality Florida — a local LGBT rights organisation — said: “We are heartbroken and angry that senseless violence has once again destroyed lives in our state and in our country.”

The Gay, Lesbian, Bisexual, Transgender Community Center of Central Florida, one of the oldest gay organisations in the US, has set up grief counselling for those affected by the tragedy, and OneBlood, an Orlando blood bank, on Sunday set up an emergency drive to collect more blood donations to help the scores of victims injured in the assault.

“We have received a steady stream of emails and messages from those seeking to help or to make sense of the senseless. We make no assumptions on motive. We will await the details in tears of sadness and anger. We stand in solidarity and keep our thoughts on all whose lives have been lost or altered forever in this tragedy,” Equality Florida said in the statement.