Mohammed al-Absi, whose work explored the corrupt relations between powerful militias and prominent businessmen, was rushed to hospital after having dinner in a Sanaa restaurant with this cousin in December.
His sudden death pressed family and friends to demand an autopsy, delaying his funeral for nearly three weeks.
Samples were flown to Jordan for tests, which finally revealed on Sunday that al-Absi died of poisoning.
In a joint statement, advocates called upon the state prosecution to start a "transparent investigation" saying that al-Absi played an "exceptional role against corruption".
No official statement has laid the blame on any group, however prominent activists and journalists who were in touch with al-Absi prior to his death, suggested he had revealed investigations looking into three oil companies owned by the Houthi leadership, including spokesman Mohammed Abdulsalam and an arms dealer from Saada, Dagsan Mohammed Dagsan.
A statement published by the team investigating the death emphasised he had "not received financial support from any party or political entity" and that the investigation into his death was funded entirely by his family and supporting human rights organisations.
The team assigned to follow up al-Absi's death includes the family of the deceased, two local NGOs, Sanaa Centre for Studies and Yemeni Journalists Syndicate.