The source added that the fact-finding committee had determined that a group accompanying Wallop travelled from Tokyo's Narita Airport to Suvarnabhumi Airport on November 14 with 30 pieces of luggage weighing a combined 170 kilograms.
Wallop's wife used her name to check in all the luggage, the source said.
Last week's report followed a tip from a THAI employee to the labour union. Wallop denied any abuse of executive power, saying the report was politically motivated.
The source said the permitted baggage weights for economy, business and first-class passengers are 20kg, 30kg and 50kg per person, respectively. Passengers are subject to excess charges if their permitted luggage weight is exceeded.
On Wednesday, a document detailing allegedly unacceptable behaviour by a senior THAI executive circulated at the carrier. The document did not name the person, but alleged that a board member of the airline had travelled with his spouse to Japan and returned on November 14 with 40 suitcases weighing a total of 520kg.
The belongings were allegedly classified as lost-and-found items to allow them to bypass Customs Department officials as they entered Thailand.
The THAI source said it would be impossible for overweight luggage to be slipped through via the lost and found, as all luggage must go through the Customs Department's tax-declaration lane.
Transport Minister Sophon Saram on Friday said he had asked Ampon Kittiampon, secretary-general of the National Economic and Social Development Board and the THAI board chairman, about the case.
He said Ampon told him the luggage at the centre of the allegations had made its way to and from the aircraft according to standard procedures and that there was no substance to any duty-avoidance claims.
The investigation committee has yet to report its results to the board.