At least one person has been arrested in the killing Tuesday of U.S. Ambassador to Libya Christopher Stevens and three other Americans, the Libyan prime minister said Thursday.

"Three or four are currently being pursued," Mustafa Abushagur told CNNI's "Amanpour." He said the arrest was made early in the day in Benghazi and that the person arrested and those being sought are all Libyans.

"The evidence itself is based on mostly pictures that were taken around the compound at that time, and also through some witnesses," he said.

The investigation is being conducted by a high-level commission, he added. "We are taking this very, very seriously. This is something which we clearly don't accept."

Earlier, the Libyan state-run news agency LANA said more than one person had been arrested. It cited the deputy minister of interior in the eastern region, Wanees al-Sharif, as its source.

The announcement came as the United States is struggling to determine whether a militant group planned the attack that killed the four Americans.

A source with knowledge of the Libya attack investigation told CNN that FBI agents were "preparing to interview witnesses" in Libya.

Conflicting theories flew in the hours after Stevens, another diplomat and two State Department security officers were killed late Tuesday in the eastern city of Benghazi.

They died amid a protest outside the U.S. Consulate over a film that ridiculed Muslims and depicted the Prophet Mohammed as a child molester, womanizer and ruthless killer.

The demonstration was one of several protests across the region that day.

U.S. sources said Wednesday the four-hour assault in Benghazi had been planned by militants, with the attackers using the protest as a diversion.

A U.S. intelligence official told CNN the picture is becoming clearer within the intelligence community as to what group or groups were responsible for the attack. Given what officials know about al Qaeda in Libya, U.S. intelligence officials believe it is very unlikely that core al Qaeda was behind the attack, the official said.

State Department Under Secretary Patrick Kennedy said Wednesday that the attack appeared planned because it was so extensive and because of the "proliferation" of small and medium weapons at the scene. He was briefing congressional staffers when he offered that theory.

But on Thursday, three U.S. officials told CNN that they have seen no evidence the attack was premeditated.

Meanwhile, Shawn Turner, director of communications for U.S. National Intelligence, denied news reports that U.S. officials had been warned of a possible attack.

"This is absolutely wrong," he said. "We are not aware of any actionable intelligence indicating that an attack on the U.S. post in Benghazi was planned or imminent."

The United States is deploying warships and surveillance drones in its hunt for the killers of the diplomatic staffers, and a contingent of 50 Marines has arrived to boost the security of Americans in the country.

The drones are expected to gather intelligence to be given to Libyan officials for strikes, the official said.

Two American destroyers were en route to the Libyan coast, U.S. officials told CNN. Both the USS Laboon and USS McFaul are equipped with satellite-guided Tomahawk cruise missiles.

The move "will give the administration flexibility" in case it opts to take action against targets inside Libya, one senior official said.

"We will not waver in our commitment to see that justice is done for this terrible act," U.S. President Barack Obama said Wednesday. "And make no mistake, justice will be done."

Obama called Libya's Mohamed Magariaf on Wednesday, thanking the newly elected president of Libya's parliament for his condolences on the deaths of the Americans.

"The two presidents agreed to work closely over the course of this investigation," the White House said in a statement. Obama "reaffirmed our support for Libya's democratic transition, a cause Ambassador Stevens believed in deeply and did so much to advance. He welcomed the election of a new prime minister yesterday to help lead the Libyan government's efforts to improve security, counter extremism, and advance its democracy."

Tuesday's attack took place on the 11th anniversary of the al Qaeda attacks on New York and Washington. But White House National Security Council spokesman Tommy Vietor said assigning any motive for the attack was "premature."

State Department spokeswoman Victoria Nuland said the evaluation of security at the Benghazi, Libya, consulate in advance of the September 11 anniversary was "appropriate for what we knew." She cited a local guard force stationed around the outer perimeters and a "robust" American security presence in the compound.