"To widen the investigation" means to increase the scope or range of the investigation by considering more factors, exploring new leads or possibilities, or gathering additional information. Full definition
The new extension will make it possible to study the auroral regions above Earth's poles and widen the investigations of the magnetosphere — its inner region in particular. (esa.int)
A huffy Mayor Bill de Blasio pleaded innocence and ignorance when reporters pressed him today about widening investigations into his fundraising and defunct political nonprofits — and he pointed to Hillary Clinton as a shining example of a politician subjected inquiries that came to naught. (observer.com)
Since last April, federal and state prosecutors reportedly widened the investigation beyond the NYPD. (silive.com)