“There are many circumstances around the theft of the Poppy Flowers that point to the fact that a museum employee participated in the theft or stole it himself,” Habib al-Adly told the official MENA news agency.
“The location and placement inside the museum confirms this,” Adly said.
“Security services are continuing to take measures internally and externally through co-operation with Interpol and several other Arab and foreign security services,” he said, adding that the theft had been a “difficult lesson”.
Vincent Van Gogh’s “Poppy Flowers”, also known as “Vase with Flowers” was stolen last month in a brazen daytime heist in a case that highlighted major security lapses in cultural institutions.
The head of the culture ministry’s fine arts sector, Mohsen Shaalan, and other senior officials are standing trial over the theft on charges of negligence.
The Dutch masterpiece, valued at more than $50m, was cut out of its frame.
Official investigations found that the museum had reduced the number of guards and that most of the surveillance cameras were not working.
Public prosecutor Abdel Meguid Mahmud has said that each painting in the Mahmud Khalil museum, which also has works by Monet and Renoir, had an alarm but that none of them worked.
The painting, of yellow and red flowers in a vase, had been stolen before in 1977, but was recovered the following year.