Rights groups have blasted right-wing US President Donald Trump's declaration that he may send the suspect of a deadly attack in Manhattan to the US-run Guantanamo Bay detention centre in Cuba.

On Wednesday, Trump announced he was considering sending Sayfullo Saipov, an immigrant from Uzbekistan who is suspected of killing eight people in Manhattan a day earlier, to Guantanamo Bay.

New York Police Department Commissioner John Miller said at a press conference on Wednesday that Saipov carried out the attack "in the name of ISIS", referring to the Islamic State of Iraq and the Levant (ISIL) group.

Within hours of the attack on Tuesday, Trump tweeted: "We must not allow ISIS to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!"

On Wednesday, the president also urged Congress to repeal the State Department's Diversity Visa Lottery Programme, which provides up to 50,000 visas a year, and replace it with a "merit-based" programme.

'Dangerous precedent'

Ibrahim Hooper, the national communications director at the Council on American Islamic Relations (CAIR), said sending the suspect to Guantanamo would "be on its face unconstitutional".

"It would set a very dangerous precedent for treating Americans differently based on faith, ethnicity or national origin and could create a two-tiered legal system," he told Al Jazeera, arguing that Trump has demonstrated a double standard when responding to violent incidents allegedly carried out by Muslims.

Speaking by telephone on Wednesday morning, Hooper alluded to a mass shooting that left 58 people dead in Las Vegas last month. In that attack, Stephen Paddock, a 54-year-old white man from the US, opened fire on a country music concert. The assailant also took his own life.

In the wake of the attack, Trump urged caution in response to widespread calls for the government to tighten gun laws. A memo distributed by the White House said that "new laws won't stop a mad man".

Hooper accused the president of "showing a difference in his approach to different incidents based on who the alleged perpetrators are" and displaying distrust in authorities' ability to execute their jobs.

On Wednesday, Trump said that the US has to "come up with punishment that is far quicker and far greater than the punishment these animals are getting right now".

He added: "We need quick justice and we need strong justice - much quicker and much stronger than we have right now. Because what we have right now is a joke and it's a laughing stock."

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Hooper said that Trump's rhetoric "seems to indicate a lack of confidence in the Federal Bureau of Investigation, the New York Police Department, law enforcement and the judicial system, which he has declared as a whole to be a joke".

The Guantanamo Bay detention camp was established by former President George W Bush in 2002 and held 684 detainees at its peak. Most prisoners are held at the facility without charge or trial. Only eight detainees have been convicted in military commissions after trail or following a plea deal, according to Human Rights Watch.

Although Bush's successor, former President Barack Obama, signed an executive order to close the camp in 2009, the facility remains open. As of January 2017, there were at least 41 detainees held in Guantanamo.

Trump has vowed to keep the detention camp open.

During the 2016 presidential campaign, then-candidate Trump said the US "was going to load it [Guantanamo Bay] up with some bad dudes".

'Neither legal nor effective'

Wade McMullen, a lawyer at the Washington, DC-based Robert F Kennedy Human Rights Center, echoed Hooper's criticisms of the president's comments on Wednesday.

"It's not even worth reacting to whether the president's suggestion of sending this suspect to [Guantanamo] is legal or even effective - it's clearly neither," he told Al Jazeera.

McMullen pointed to Trump's response to a deadly attack at a white supremacist rally in Charlottesville, Virginia, on August 12.

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During that incident, James Alex Fields, who had been photographed marching with neo-Nazis earlier in the day, allegedly ploughed his car into an anti-racist march, killing 32-year-old activist Heather Heyer and injuring 19 others.

Although Trump subsequently denounced the attack and disavowed a host of white supremacist organisations that participated in the rally, he insisted that "there were very fine people" among both the white supremacists and the counterdemonstrators.

"What we should be asking is why is this president is quick to use such radical, extreme language in the wake of the tragedy in New York, but after the terror attack in Charlottesville, could only muster something about how 'there were very fine people on both sides'," McMullen said.

"Racism and Islamophobia will never result in sound, legal or effective counterterrorism policy."

The American Civil Liberties Union (ACLU), another rights organisation, also condemned Trump's rush to impose sweeping measures in the wake of the attack.

"Sending Saipov to Guantanamo or treating him as an ‘enemy combatant’ would violate due process and the rule of law," Anthony D Romero, executor director of the ACLU, said in a statement on Wednesday.

"It's a shame that Trump is using this attack as a platform for pushing his anti-immigrant, anti-Muslim agenda."