UNITED NATIONS -- The United States has informed the United Nations Security Council that a top suspect in a deadly 2012 assault in Benghazi, Libya, was captured because U.S. investigators determined that he planned further attacks against Americans.

In a letter to the council obtained Wednesday by The Associated Press, U.S. Ambassador Samantha Power said the capture of Ahmed Abu Khattala was necessary "to prevent such armed attacks" and was carried out "in accordance with the United States' inherent right of self-defense."

She called Abu Khattala "a senior leader of the Libyan militant group Ansar al-Sharia-Benghazi in Libya."

Power said U.S. investigators determined that Abu Khattala was "a key figure" in the Benghazi attacks that killed four Americans including Ambassador J. Christopher Stevens. She said Abu Khattala will be prosecuted in a U.S. federal court.

Abu Khattala may be taken to the United States aboard the Navy ship where he is being held, U.S. officials said Wednesday, a prospect that would probably extend the time he can be interrogated by the FBI without being taken before a court.

Although the federal court where he is to be tried has not yet spoken on the issue, it could object to the relatively slow mode of transport, officials said. Abu Khattala is aboard the amphibious transport ship USS New York in the Mediterranean Sea.

The administration has said only that he will appear before the U.S. District Court in Washington "in the coming days." Should the court want him here more expeditiously, he could be transferred off the ship and taken to this country by air.

FBI interrogators, probably acting in part under a public safety exception, have not yet informed Abu Khattala of his Miranda rights to silence and an attorney, according to several U.S. officials, who spoke on the condition of anonymity about intelligence and legal matters related to the case.

Questioning under the exception could go on for hours or days, legal experts said. But the longer the government relies on it, the greater risk that a judge may eventually prevent prosecutors from using some of his statements in criminal court.

That is a risk that the government may be willing to take to gather other intelligence, former government lawyers said, if law enforcement believes it already has enough evidence to make its case against Abu Khattala without information gleaned from a lengthy interrogation.

The absence of a formal indictment against him, experts said, also gives law enforcement additional leeway to investigate before presenting his case to a grand jury. A sealed criminal complaint filed last July charged him with three counts of involvement in the September 2012 attacks on U.S. diplomatic and CIA installations in Benghazi that killed the four Americans.

U.S. officials have said the joint military-FBI operation that apprehended Abu Khattala south of Benghazi last weekend was months in the planning. Success was facilitated, a senior administration official said, by "a new opportunity that did not exist before."

The official declined to specify the nature of the opportunity, but Libyan militia members and neighbors of Abu Khattala in Benghazi have said that while he normally was on the move and difficult to locate in recent weeks, he had returned unaccompanied to his home neighborhood south of the city Sunday evening.

Apprehended inside a building, rather than at his home, as previously reported, Abu Khattala put up some initial resistance and suffered what officials called "minor" injuries while being taken into custody. The Pentagon has said there were no casualties during the operation.

The Libyan government Wednesday denied it had prior knowledge of Abu Khattala's capture and demanded his return.

"The government stresses its right to try Abu Khattala on its territories and according to its laws," a Libyan statement said.

"It also demands his safety be ensured," it added, saying the man was also wanted by Libyan authorities, but because of the volatile security situation in Benghazi they had not been able to arrest him.

Information for this article was contributed by staff members of The Associated Press and by Karen DeYoung, Ann E. Marimow and Adam Goldman of The Washington Post.

A Section on 06/19/2014