The phrase "are speculated to have" is correct and usable in written English.
It can be used when discussing theories or assumptions about past events or conditions that are not definitively proven.
Example: "The ancient ruins are speculated to have been part of a larger civilization that once thrived in the region."
Alternatives: "are believed to have" or "are thought to have".
The Blairs are speculated to have bought the sketch, but the gallery is keeping mum.
Due to evolutionary divergence, cattle (taurine, and indicine) and buffalo are speculated to have different responses to heat stress condition.
Repeated depositions of seismo-turbidites due to similar remobilizations of surface sediments are speculated to have occurred along the Japan Trench.
As both VKH and SO are speculated to have an underlying T cell-mediated autoimmunity to uveal/retinal antigen, the melanin-bearing hair follicles may be targeted if the inflammation was not controlled well enough.
They are speculated to have plans for as many as 120 more.
Both breeds are speculated to have stemmed from the Chien d' Oysel described in the writings of Gaston Phoebus.
Besides, the carbon in solid solution is speculated to have the effect on the refinement of α2 phase.
August 21 , 2013 KSS is speculated to have been involved in the Assad Regime's chemical weapons attack on the Damascus suburb of East Ghouta.
In addition to the large specific surface area, the hydrogen spillover effect is speculated to have accelerated the electrooxidation rate of formic acid in the direct pathway.
While the origin of carbon-based life on Earth is speculated to have begun ~ 3.8 billion years ago, carbon is of greater antiquity and its existence transcends into the primeval epochs of the universe.
Because of free gas migration in sediment pores, the Shenhu seafloor layers in the northern South China Sea is speculated to have experienced some rapid methane hydrate generation and accumulation.
