The phrase "further interrogation" means asking more questions or continuing to investigate someone or something in order to gather more information or details. Full definition
He was later handed over to the Airport police for further interrogation. (ghanapoliticsonline.com)
That upon further interrogation they admitted that they committed the robbery of the items belonging to justice Getrude Araba Torkonoo, Francis Torkonoo, Anthony Koduah Bediako and Patience Hammond. (ghtvonline.com)
This aspect needed further interrogation into what it was about the teacher that made the experience notable. (teachermagazine.com.au)