Mr Solanki was interrogated by the CBI today in Delhi.
Mr Jethwa was shot dead in 2010 in Ahmedabad by two men on a motorcycle near the Gujarat High Court. The 34-year-old was headed out of his lawyer's office when he was killed.
He had accused Mr Solanki of being involved in illegal mining in and around the Gir Forest area in Gujarat.
Mr Jethwa's family has said he was killed for combatting the powerful mining mafia in Saurashtra.