Obama told reporters Tuesday that Libi -- a computer expert charged in a New York court over the 1998 bombings of the US embassies in Kenya and Tanzania -- "planned and helped to execute a plot that killed hundreds of people, a whole lot of Americans. "
"We have strong evidence of that. And he will be brought to justice," Obama said, vowing to go after "active networks" that aim against the United States.
His administration justified the operation under the 2001 Authorization for Use of Military Force by the US Congress, which authorized the president to target any nation, group or person involved in the September 11 attacks.
But the case under international law is less clear. Article 2(4) of the UN Charter bars nations from the threat or use of force against other states.
The United States may have received tacit approval from Libya, despite its public denials, but otherwise would need to offer another rationale such as that Libya was allowing an attack to be planned on its territory, said Robert F. Turner, a professor at the University of Virginia School of Law.
"It would not surprise me at all if we had the private consent of the authorities and, if that's the case, it was a perfectly lawful act," Turner said.
"It would also be a lawful act -- although controversial -- if he were still involved with terrorism," he said.
But Turner said it was critical for the United States to provide a legal justification.
"If we start saying our obligations don't count because we're the biggest gorilla in the zoo, we're not going to have a strong case to tell the Iranians or North Koreans or anyone else that they need to observe their non-proliferation obligations," he said.
In 2003, the CIA snatched the cleric Mustafa Osama Nasr on the streets of Milan and sent him to his native Egypt, where his lawyers said he was tortured. An Italian court later sentenced 23 CIA agents to prison in absentia, despite speculation that then prime minister Silvio Berlusconi had quietly approved the operation.
Still seeking to close Guantanamo
US authorities are likely to send Libi to face trial after he is interrogated aboard the warship, reportedly the USS San Antonio.
In a prior precedent for the Obama administration, Ahmed Abdulkadir Warsame -- a Somali with alleged Al-Qaeda ties -- was brought to New York for trial in 2011 after being interrogated for more than two months aboard the USS Boxer.
"The same kinds of concerns and issues arise here in that the administration seems to be relying on law of war rationales to escape the constraints that apply under the criminal justice system," said Hina Shamsi, director of the American Civil Liberties Union's National Security Project.
Article 22 of the Third Geneva Convention on the rules of war forbids holding prisoners on boats. But the Bush administration began arguing that terrorism suspects were not prisoners of war.
In one of its most controversial decisions, the Bush administration sent Al-Qaeda suspects to the US naval base in Guantanamo Bay, Cuba, where they would not be entitled to protections guaranteed by the US Constitution such as habeas corpus -- the requirement that any detainee is brought before a judge.
Matthew Waxman, a professor at Columbia Law School, said that while the 2001 authorization of force had plenty of gray areas, the case that Libi was related to Al-Qaeda "seems relatively strong."
The Obama administration's justifications "are very similar to arguments that the Bush administration relied on, though the Obama administration has demonstrated a much stronger interest in bringing such suspects to trial in the United States rather than detaining them long-term in Guantanamo or other foreign sites," he said.
Obama promised on taking office in 2009 to close the prison at Guantanamo Bay within a year. But he failed, with the rival Republican Party blocking attempts to transfer inmates and few other countries wanting to take in the men even if they are cleared of wrongdoing.
Republican Senator Lindsey Graham called Tuesday on Obama to transfer Libi to Guantanamo for interrogation before a trial in a US court, which the Obama administration has already ruled out.