U.S. Sen. Elizabeth Warren (D-Mass.) expanded her inquiry of bitcoin miners’ energy usage and their environmental footprint, sending letters to six more miners on Thursday.

Warren wrote to Riot Blockchain, Marathon Digital Holdings, Stronghold Digital Mining, Bitdeer, Bitfury Group and Bit Digital questioning their “extraordinarily high energy usage.”Last December the Massachusetts senator, who has made environmental issues a focus of her office, sent a letter to the bitcoin miner Greenidge, expressing her concerns about its high energy usage.In the new letter, Warren and her colleagues asked each miner to detail electricity consumption, scaling plans, agreements with electricity companies and impact on energy costs for consumers and small businesses by Feb. 10.“The extraordinarily high energy usage and carbon emissions associated with Bitcoin mining could undermine our hard work to tackle the climate crisis – not to mention the harmful impacts cryptomining has on local environments and electricity prices,” Warren said in the letter.The correspondence adds to a list of inquiries by lawmakers globally on the energy consumption of proof-of-work (PoW) mechanisms that reward crypto miners for validating transactions.Most recently, the U.S. held a congressional hearing to discuss the energy consumption related to the PoW. Meanwhile, the EU markets regulator called for a ban on the validation system, citing its energy intensity.