As the charges are federal, the government can override NY state's ban on capital punishment.
Numerous legal analysts said that, given his position in the government, Trump should not be pre-judging the outcome of any trial; defense lawyers will no doubt seize on the president's comments to claim the trial can not be fair.
John Miller, deputy NY police commissioner for intelligence, said Saipov appears to have followed the instructions issued by Islamic State. He said the yet-to-be-assigned judge will question prospective jurors to ensure they can be fair despite anything they might have heard or read. "Should move fast. DEATH PENALTY!".
Trump has also been critical lately of the United States justice system, saying it takes too long to punish criminals.
"The terrorist attack in NY is the latest brutal, horrific example of the war that radical Islamist extremists are waging against our nation and our way of life", McCain said in a statement Wednesday.
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Saipov did not ask for bail and was remanded to federal custody.
Tuesday's assault was the deadliest in New York City since September 11, 2001, when suicide hijackers crashed two jetliners into the World Trade Center, killing more than 2,600 people. His attorney, David Patton, has said he hopes "everyone lets the judicial process play out".
"Mr. President, we all know he should get the death penalty".
"Would love to send the NYC terrorist to Guantanamo but statistically that process takes much longer than going through the Federal system", Trump has stated in a tweet. A 23-year-old New York City man, a 32-year-old New Jersey man, five Argentinians and a Belgian national were among the eight people who died.
"We're so politically correct that we're afraid to do anything", he said.
However, an administration official told NBC News that there's likely no legal authority to send Saipov to the detention facility.