Russian police said on Monday they planned to use a lie detector to question witnesses about the vicious acid attack against the artistic director of the Bolshoi ballet.
"Investigators... plan to question several witnesses in the case of the attack on Sergei Filin with a lie detector," a spokesman with the Moscow police told Russian news agencies.
Last week police questioned Bolshoi's star dancer Nikolai Tsiskaridze as a witness over the January 17 attack by an unknown assailant who fled after throwing acid into Filin's face and eyes.
The violent incident came after months of squabbling inside what is arguably the world's best known ballet troupe.
A former star dancer himself, 42-year-old Filin is currently in hospital in Moscow and has undergone several operations to save his eyesight and repair the disfigurement he suffered.
Doctors said on Monday his latest eye surgery went well and some vision has returned, but more treatment was necessary.
"Today he has eyesight. I won't say how much, but he sees relatively well after such a (grave) injury," Russia's head ophthalmologist Vladimir Neroyev told Interfax news agency.
The attack on Filin, who has served as artistic director of the Bolshoi ballet since 2011, has horrified the troupe and cast an international spotlight on the sometimes scandalous rivalries within the 237-year-old institution.
Bolshoi's management has linked the attack to internal divisions within the company, while other observers, including Filin's predecessor Alexei Ratmansky, blamed the Bolshoi's lack of "theatre ethics" and unresolved money-related issues such as ticket scalping.