Jethwa had filed several RTI queries and also a PIL in Gujarat High Court with regard to illegal mining in Gir forest, the abode of Asiatic lion.
Solanki was arrested by CBI in Delhi earlier this month for his alleged involvement in the killing of the activist and was brought to city the next day on a transit remand.
CBI, which had initially sought his 14-day custodial remand for further interrogation, was granted four-day remand.
ACJM Khutwad today extended Solanki's judicial custody till December 9.
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The case was initially probed by Gujarat police, who had arrested six persons, including Shiva Solanki, nephew of the MP, and alleged sharp-shooter Sailesh Pandya who is accused of gunning down Jethwa outside the Gujarat high court on July 20, 2010.
However, the police had ruled out the possibility of Solanki's involvement in the crime and gave him a clean chit, following which the slain activist's father approached the high court.