Well, certainly by the French ambassador.
The only theory that actually makes sense is that Henry Stafford, Duke of Buckingham killed them (whom Richard had executed)
And it makes no sense at all that Richard ordered them killed to rid himself of a rallying point for his opposition? The conspirators in the 1483 Rebellion (for which Buckingham was executed) started off to put Prince Edward on the throne, but when it became common knowledge that he and his brother were already dead, they switched their allegiance to Henry.
“And it makes no sense at all that Richard ordered them killed to rid himself of a rallying point”
The princes weren’t a rallying point for the Lancastrians. They were York princes. They were an obstacle.
“they switched their allegiance to Henry”
The only document pre 1485 states that Buckingham was responsible for the death of the princes of the tower. He was also guard of the Tower while the brothers were held. His lineage was also senior if, and only if the brothers died.
The French Ambassador was where Henry was in exile. One guess why he would side with Henry. ;)