Senators from both parties accused the Defense Department on Wednesday of obstructing an investigation into whether a highly classified intelligence program known as Able Danger did indeed identify Mohamed Atta and other future hijackers as potential threats well before the terrorist attacks of Sept. 11, 2001. The complaints came after the Pentagon blocked several witnesses from testifying before the Senate Judiciary Committee at a public hearing on Wednesday. The only testimony provided by the Defense Department came from a senior official who would say only that he did not know whether the claims were true. But members of the panel, led by Senator Arlen Specter, Republican of Pennsylvania, said they regarded as credible assertions by current and former officers in the program. The officers have said they were prevented by the Pentagon from sharing information about Mr. Atta and others with the Federal Bureau of Investigation. The Pentagon has acknowledged that at least five members of Able Danger have said they recall a chart produced in 2000 that identified Mr. Atta, who became the lead hijacker in the Sept. 11 plot, as a potential terrorist.