Gwanda's wife said he vanished on November 21, after he left in a white Toyota Land Cruiser with unknown people on an "emergency trip", telling her he would return the following evening.
"She became alarmed after he failed to return home and all his numbers could not be (reached) since the day he departed," the newspapers said in a statement on Monday.
Police are investigating the case. The newspapers, part of the East African Nation Media Group, called on them to "speed up the investigation."
Police say they have now broken up the gang of motorcycle assassins.
The motives behind the attacks remained unclear, although President John Magufuli hinted in June the attacks might have been religiously motivated, with some blaming Islamic extremists.