People say that Indica Indians of Peru were the first people to grow potatoes as a crop.
As I understand it, the passive tense is:
It's said that potatoes were first grown as a crop by the Indica Indians in Peru.
But why in this sentence we say 'were' - past simple, whereas in the following sentence:
The prince is believed to have been assassinated in his bedroom.
we use, 'have been' instead of 'was' assassinated.