The Solomon Islands police are challenging the validity of a permit that led to immigration charges being dropped against the suspended Attorney General, Julian Moti.
The Australian lawyer was facing two charges after flying into the Solomons from Papua New Guinea on a clandestine military flight in October.
The magistrates court in Honiara this week dropped the charges against Mr Moti of illegal entry and not having a valid passport.
The move came after Mr Moti produced as evidence a permit to enter and reside in Solomon Islands, which the court deemed to be a valid document.
The police commissioner, Shane Castles, says they had sought an adjournment of the case to investigate the document, which they believed was invalid.
"We the police believe the document is suspicious and we intend to investigate its authenticity and also to have it forensically examined. It's the first time its surfaced this week and if we go back to the 10th of October when I met with immigration officials they told me that there had previously been a permit, but that permit had expired."
Mr Castles says the document was historical and had purportedly been renewed.
Meanwhile the Australian government is still pushing ahead with plans to extradite Mr Moti on child sex charges relating to incidents in Vanuatu in 1997.