U.S. forces interrogating some Iraqi prisoners are seeking details about the case of Navy pilot Michael Scott Speicher (search), missing since his fighter jet was shot down on the opening night of the 1991 Persian Gulf War, Defense Secretary Donald H. Rumsfeld (search) said Tuesday.

Rumsfeld would not say whether there were any new clues to the fate of Speicher, the only American serviceman still listed as missing from the Gulf War (search).

"There is nothing that has been turned up thus far that I could elaborate on that would be appropriate," Rumsfeld told a Pentagon news conference.

Members of the Iraq Survey Group, the 1,400-member team searching for weapons of mass destruction and top former Iraqi leaders, are trying to track down Speicher, Rumsfeld said.

Sen. Bill Nelson, D-Fla., who has been pushing for an aggressive investigation into Speicher's fate, said he welcomed news of the interrogations. Nelson discussed the matter with Rumsfeld Tuesday during a visit of senators to the Pentagon.

"This is what they should have been doing all along," Nelson said.

Nelson said there were no breakthroughs in the search for Speicher, and any new details on the case were classified.

In April, American investigators found Speicher's initials -- "MSS" -- carved into the wall of a cell in an Iraqi prison. U.S. officials said they had other evidence Speicher may have been held at the Hakmiyah prison, but added that Saddam Hussein's regime frequently moved prisoners of war.

Speicher's F/A-18 Hornet was shot down over Iraq on Jan. 17, 1991, the first night of the Gulf War. Months later, the Pentagon listed him as killed in action, although his remains have never been recovered. Last year, the Defense Department changed his status to "missing-captured" amid new intelligence about his possible survival.

Saddam's government said Speicher was killed and denied holding him as a prisoner.