The lawyers for accused insider trader Raj Rajaratnam, the head of the Galleon Group hedge fund, launched their first attack on the government’s case yesterday, claiming the chief witness in the $25 million insider-trading prosecution was a liar who could not be trusted.

John M. Dowd and his legal team at Akin Gump Strauss Hauer & Feld said that cooperating witness Roomy Khan was a convicted felon who was “adjudicated by a court in August 2009 to have fabricated evidence in a court proceeding.”