Former prime minister Benazir Bhutto was assassinated in a gun and bomb attack in Rawalpindi on December 27 in 2007 after she addressed a public meeting in the garrison city.
Siegel, an American journalist, has said that he could not arrive Pakistan due to security concerns but was ready to testify in the case through video link.
A Rawalpindi anti-terrorism court (ATC) on Monday summoned Siegel to record his statement in Benazir Bhutto murder case.
ATC judge Pervez Ismail Joeya directed the American lobbyist to appear before the court to record his statement on Tuesday.
According to the investigation sources Siegel was the key witness against former military ruler Pervez Musharraf in the case.
Siegel in his statement recorded in 2009, had accused Musharraf of threatening Bhutto about her safety in Pakistan.