Sentence:  During the course of his looking into Abedin's S.G.E. status, the senator <WSD>stumbled upon</WSD> an O.I.G. “criminal” inquiry, commenced in October 2013, about whether Abedin knowingly got paid for hours she did not work while she was on vacation and maternity leave. 
Meaning of the word marked with <WSD> tags:  To discover or find something by accident.