Can the NYC terrorist suspect be detained in Guantanamo Bay?

— -- President Donald Trump appeared to be moving away from his previous suggestion that he would consider sending the suspect in the recent New York City terrorist attack to the Guantanamo Bay detention center in Cuba.

Trump’s main qualm with sending the suspect, Sayfullo Saipov, to Guantanamo Bay is that prosecutions there take too long — even longer than criminal proceedings in federal civilian courts. Saipov is accused of driving a rented truck onto a bike path in lower Manhattan, killing eight people and injuring a dozen more.

“Would love to send the NYC terrorist to Guantanamo but statistically that process takes much longer than going through the Federal system…” Trump tweeted Thursday.

He added: “There is also something appropriate about keeping him in the home of the horrible crime he committed. Should move fast. DEATH PENALTY!”

There are unanswered legal questions about the Constitutional limit on the government’s power to detain Saipov at Guantanamo since he holds a U.S. green card.

Further, prosecutions through the military system take longer than criminal proceedings in federal civilian courts. Charges have already been filed in the Southern District of New York court against Saipov for providing support to ISIS and violence and destruction of motor vehicles. He appeared for his arraignment in a wheelchair Wednesday.

Add to that the political controversy that continues to surround the use of Guantanamo Bay for detainees suspected of terrorism and the likely backlash against moving Saipov there.

For those reasons and myriad more, even with the potential backing of the president, it would be difficult for the U.S. to send Saipov to Guantanamo Bay.

Here’s what you need to know about the process:

What is Guantanamo Bay?

U.S. Naval Station Guantanamo Bay is America's oldest overseas military installation, established in southeast Cuba in 1903. Separate from the Navy base, there is a detention facility established in early 2002 to house enemy combatants captured overseas.

Why are detainees at Guantanamo?

The Bush administration established the detention facility there so that enemy combatants would not be subject to U.S. laws. At Guantanamo, there is a separate military commission process to try detainees. However, only a fraction of them have ever faced criminal prosecution.

How many detainees remain at Guantanamo?

There are currently 41 detainees still at Guantanamo. The peak number of detainees at Guantanamo was about 800 during the Bush administration. More than 500 detainees were transferred out during the Bush administration another 242 were transferred out during the Obama administration.

Who are the remaining detainees?

Five of the 41 remaining detainees at Guantanamo have been approved for transfer to third-party countries. Ten of the remaining detainees are currently in different stages of prosecution including Khalid Sheikh Mohammed, the mastermind of the September 11, 2001, terror attacks. Another 26 detainees are eligible for Periodic Review Boards that determine if they continue to pose a terror threat to the U.S. However, it is often assumed that these detainees will neither be eligible for prosecution nor transfers.

When was the last time a detainee arrived at Guantanamo?

The last detainee arrived in Guantanamo in March, 2008.

Wasn’t the detention facility at Guantanamo supposed to close?

In his first week in office in January 2009, President Obama signed an executive order that designated the detention facility at Guantanamo to close within a year’s time. But that goal was never reached because of congressional opposition throughout the Obama administration. Eventually, many Guantanamo detainees were deemed eligible for transfer as part of an inter-agency review that determined if they still posed a threat to the United States. But the limiting factor for the Obama administration was finding countries willing to take the transferred detainees.

What is an enemy combatant?

An enemy combatant is an unlawful enemy of the state who engages in military conflict. The Bush administration used this category to address individuals caught by the military who as they engaged in either direct combat or plotted terror attacks against the United States. The Justice Department has the authority to designate individuals as enemy combatants who could be prosecuted through a military commissions process at Guantanamo. The Trump administration has revived the term after the Obama administration withdrew its use beginning in 2009.

Can Saipov be tried in Guantanamo Bay?

During the presidential campaign, Trump said he intends to keep the detention facility open and "load it up with some bad dudes."

The government bases its authority to hold detainees at Guantanamo on the 2001 Authorization for the Use of Military Force, which allows for the use of force against those that the president determines had some involvement in the 9/11 attacks that killed nearly 3,000 Americans.

Saipov is a 29-year-old immigrant from Uzbekistan who entered the U.S. in 2010 and later became a legal permanent U.S. resident. The detainees in Guantanamo Bay are not U.S. terror suspects and federal law currently prohibits U.S. citizens from being tried in military tribunals.

The White House has said that the Trump administration considers Saipov an “enemy combatant.”

For the past several years, terrorism suspects arrested inside the United States and overseas have uniformly been brought to U.S. cities for prosecution in federal civilian courts.

According to Stephen Vladeck, who is a law professor at the University of Texas, a main legal obstacle to detaining Saipov in Guantanamo is that he is a “green card” holder and is legally in the United States. The question is, Vladeck said, “might there be a Constitutional limit on the government’s power to detain him?”

Another challenge in trying to detain Saipov in Guantanamo is that the courts have not settled whether the 2001 AUMF applies to the Islamic State, which was formed officially years after 9/11. The government will have prove that Saipov is really a member of ISIS as opposed to someone who just shares their views. Also, Saipov has also already been indicted, and quashing an indictment only leads to more problems down the road, according to Vladeck.

Does the federal court system move more quickly than the military commissions?

According to the independent advocacy organization Human Rights First, the federal courts have convicted over 600 individuals charged with terrorism offenses since September 11, 2001. In the last fifteen years, military commissions at Guantanamo have produced only eight convictions.

“There are all kinds of novel, procedural, evidentiary, logistical and jurisdictional questions that the commission take forever to sort through versus our established civilian courts which don’t have to answer those questions,” Vladeck told ABC News.

Trump's earlier assertion that the federal process moves more quickly than military commissions is not new.

In late 2009, President Obama's then-Attorney General Eric Holder announced plans to prosecute five alleged 9/11 co-conspirators, including Khalid Sheikh Mohammed, in a New York federal court.

“After eight years of delay, those allegedly responsible for that attacks of September the 11th will finally face justice,” Holder said on Nov. 13, 2009. “They will be brought to New York -- to New York -- to answer for their alleged crimes in a courthouse just blocks away from where the Twin Towers once stood.”

However, in a reversal, the Obama administration announced on April 2011 that Mohammed and the four co-conspirators would be tried “in a military commission,” with Holder blaming Congress for stonewalling their efforts to try them in a federal court.

In 2013, ABC News asked Holder if he thought the prosecution of Mohammed could have moved more quickly in a civilian court than through the military. Holder stood by his decision, but added, "If you look at the history of Article 3 prosecutions, you will see that they don't take nearly as long as those that occur in the military system."

He also said he believed that all five terror suspects "would be on death row as we speak" if politics hadn't gotten in the way.

On Thursday, Holder weighed in on Twitter, saying federal courts are the "swiftest" and "most effective" form of justice for terrorist suspects.

He also responded on Twitter to Trump’s suggestion Thursday: "Those who criticized my decision to try 9/11 plotters in federal system take note. I was right. 16 years-no justice. Use courts-not Gitmo."

Trump said Wednesday that the U.S. needs "quick justice and we need strong justice -- much quicker and much stronger than we have right now.”

He added, “Because what we have right now is a joke and it's a laughingstock.”

White House press secretary Sarah Sanders clarified that Trump was “simply pointing out his frustration of how long that this process takes” for a suspected terrorist.

ABC News’ Mike Levine contributed to this report.