The Killer's Memo

The legendary detective tandem Leonov — Makeev.
 
Three corpses with signs of poisoning have been found one after another in Moscow. The first victim, a former prima ballerina of the Bolshoi Theatre, was discovered by her tenant. The woman was sitting... in a theatrical pose in an old armchair, seemingly looking out the window. The second victim, a mathematics professor, was found on a bench near his house. The third — a young girl — was found under the parapet of the embankment with a bouquet of white lilies. All of them held notes addressed to Colonel of the Ministry of Internal Affairs Lev Gurov. The detective understands that these crimes are a challenge specifically for him. But what is the unknown killer hinting at with these scenes?
 
It seems that there are unpaid debts in the criminal practice of the operative...