The man who died after holding four people hostage at a Texas synagogue in what President Joe Biden called an "act of terror" was identified by the FBI on Sunday as a 44-year-old British citizen named Malik Faisal Akram.

The four hostages -- including a respected local rabbi, Charlie Cytron-Walker -- were all released unharmed Saturday night, prompting relief in the United States, where the Jewish community, Biden renewed calls to fight anti-Semitism.

There was "no indication" that anyone else was involved in the attack on the Congregation Beth Israel synagogue in the small Texas town of Colleyville, the statement from the FBI's field office in Dallas said.

It did not give any further information about Akram or what his motive may have been.

Biden declined to speculate on the motive but appeared to confirm US media reports that the hostage-taker was seeking the release of convicted terrorist Aafia Siddiqui, a Pakistani neuroscientist known as "Lady Al-Qaeda."

"This was an act of terror" that was related to "someone who was arrested 15 years ago and has been in jail for 10 years," Biden said in comments to reporters during a visit to a hunger relief organization in Philadelphia.

Britain's foreign minister Liz Truss likewise Sunday condemned the hostage-taking as an "act or terrorism and anti-Semitism."

Siddiqui, the first woman to be suspected by the United States of links to Al-Qaeda and a cause celebre in Pakistan and in South Asian jihadist circles, was detained in Afghanistan in 2008.

Two years later she was sentenced by a New York court to 86 years in prison for the attempted murder of US officers in Afghanistan.

Siddiqui is currently being held at a prison in Fort Worth, Texas. Her lawyer has said she "has absolutely no involvement" in the hostage situation and condemned it.

Any links she may have to Akram remained unclear.

Police have not said whether the assault team killed Akram or whether he killed himself.

FBI special agent Matthew DeSarno told reporters in Colleyville on Saturday, after the climactic end of the standoff, that the investigation would "have global reach."

"We have been in contact with multiple FBI leads to include Tel Aviv and London," he said.

He said the suspect's demands were "focused on one issue that was not specifically threatening to the Jewish community." 

Britain's ambassador to the United States confirmed that British authorities were "providing our full support to Texas and US law enforcement agencies."

"We stand with US in defending the rights and freedoms of our citizens against those who spread hate," Truss, the British foreign secretary, posted on Twitter.

 - 'Mind-blowing' -

Residents of Colleyville, about 25 miles (40 kilometers) northwest of Dallas, were still struggling to comprehend the incident on Sunday morning.

"Colleyville ... It's one of the safest towns in North Texas," said Austin Sewell, owner and founder of the North Texas Kings baseball club, whose field is across the street from the synagogue in a quiet, residential neighborhood.

"It's mind-blowing, to be honest," he told AFP.

Others took a harder line.

"That's the way it should have ended. That guy didn't deserve to live. Come here and do that... I don't like that at all. I like that ending," 91-year-old Bob Fitzgerald told AFP as he arrived to attend the Sunday service at a nearby Baptist church. 

At one point the standoff involved some 200 local, state and federal law enforcement officers massed around Colleyville. They included an FBI team flown in from Washington.

A Facebook livestream of the congregation's Shabbat service appeared to capture audio of a man talking loudly, but did not show the scene inside the building.

He could be heard saying, "You get my sister on the phone" -- apparently using the word "sister" figuratively -- and "I am gonna die."

He was also heard saying: "There's something wrong with America."

In Philadelphia, Biden said Akram was armed with weapons he "allegedly" purchased "on the street."

He said Akram had not been armed with any bombs, despite media reports.

One hostage was freed early in the standoff. 

Hours later, after police said were extensive negotiations, an elite SWAT team burst into the synagogue and the remaining three hostages were freed.

Journalists nearby said they heard a loud bang -- likely a flash-bang grenade used as a distraction -- and shots.

The siege had sparked an outpouring of concern from Jewish organizations in the United States.

Biden pledged to "stand against anti-Semitism and against the rise of extremism in this country."