The grenade landed on the fourth floor of Building 1 inside the NACC compound on Sanambin Nam road in Nonthaburi province.
The front gate of the NACC was sealed off and occupied by redshirt supporters of the caretaker government.
They seized the NACC head-office to demand the graft buster to stop work and protest the probe of the caretaker Prime Minister Yingluck Shinawatra on malfeasance and corruption charges involving the controversial fake rice sale to China.
The second attack which hit the building inside was the second time in less than a week.
EOD police believed the grenade was fired from a remote Soi Nonthaburi 48 which is only 50 metres off the wall fence of the NACC office, and about 200 meteors from where the redshirts were holding out and stayed overnight.
The NACC office was hit by M79 grenade on March 24. Earlier, hand grenade was all thrown but landed outside the fence.
Nonthaburi governor and provincial police chief rushed to inspect the scene last night with the Explosive Ordnance Disposal (EOD) police.
Political motive was a strongest motive in the attack.
The NACC yesterday rejected Ms Yingluck’s request for an extension of 45 days to report the NACC March 31 to defend charges.
Her redshirt supporters also called on all the NACC board to resign claiming it unfairly treated her.
Latest report said the police arrested a group of men who they suspected might be responsible for the grenade attack. Investigation is still underway.