Senate Judiciary Committee Chairman Arlen Specter (search) said Wednesday he wants answers from the Defense Department about Able Danger, a secret military unit that is said to have identified four of the Sept. 11 hijackers more than a year before the terrorist attacks.

Pentagon officials blocked five key witnesses from testifying in the Able Danger (search) hearings on Capitol Hill Wednesday, citing security concerns.

"I think the Department of Defense owes the American people an explanation about what went on here," Specter said. "The American people are entitled to some answers."

The testimony was expected to offer information on the secret military unit and its identification of Mohamed Atta (search) — the lead hijacker during the Sept. 11 terror attacks.

Lt. Col. Anthony Shaffer (search) said in written testimony through his attorney that the Pentagon refused to allow him to testify before the committee. His attorney, Mark Zaid, also said that the Pentagon prevented testimony from a defense contractor that he also represents.

Bryan Whitman, a Pentagon spokesman, said that the Defense Department had a representative at the hearing and that it had provided sufficient information to committee members.

“I think there are aspects of this as a classified program that we have expressed some concerns with respect to the appropriateness of some things in an open hearing,” Whitman told reporters on Wednesday. “We are working very closely to provide all the information that [committee members] need to assess Able Danger.”

Zaid fielded questions from committee members on behalf of Shaffer and contractor John Smith. He testified that Able Danger, using data mining techniques, identified four of the terrorists who struck on Sept. 11, 2001.

Zaid said Shaffer would have testified about charts his team created dealing with Al Qaeda (search) and a grainy photo on file of Atta.

“Shaffer remembers it specifically because of the evil death look in Mohamad Atta’s eyes,” Zaid said.

Pentagon officials had acknowledged earlier this month that they had found three people who recall an intelligence chart identifying Atta as a terrorist prior to the Sept. 11 attacks.

Specter asked the official representing the Department of Defense at the hearing, William Dugan (search), the acting assistant to the secretary for intelligence oversight, if the department had any information about an Al Qaeda cell and Atta.

"I don't know," Dugan replied.

Specter asked Dugan to "find out the answers to those questions" relating to what the department knew about the workings of Able Danger.

Able Danger personnel have said they tried to give the FBI information three times, but Defense Department attorneys refused, citing legal concerns about investigations run by the military on U.S. soil, Zaid said.

Former Army Major Eric Klein Smith also testified that he was instructed to destroy data and documents related to Able Danger in May and June of 2000, in accordance with Army regulations that limited the collection and holding of information of U.S. persons.

Klein Smith said the order to destroy data was not hostile or aggressive, it was a matter of policy. Asked if this information could have prevented Sept. 11, the major said he could not speculate, but believed it would have been significant and useful.

Klein Smith said that he did not remember seeing a picture of Atta, but said he believed "implicitly" claims by Shaffer and Navy Cpt. Scott Phillpott that they had seen Atta's picture. Phillpot was not on the witness list for Wednesday's hearing.

Zaid told committee members that some of the secret unit's records were also destroyed in March 2001 and spring 2004.

Rep. Curt Weldon (search), R-Pa., was the first lawmaker to come forward with claims that the Sept. 11 commission that investigated pre-attack intelligence failed to accept offers from Able Danger staff about the data it had before the attacks.

Weldon said their refusal to hear from Able Danger's members makes the government record of intelligence incomplete.

Former members of the Sept. 11 commission have dismissed the Able Danger findings.

FOX News' Melissa Drosjack and Nick Simeone and The Associated Press contributed to this report.