The Lestranges and Barty Crouch Junior tracked down Frank Longbottom in the belief that he knew the "present whereabouts" of Voldemort.
"We have heard the evidence against you. The four of you stand accused of capturing an Auror - Frank Longbottom - and subjecting him to the Cruciatus Curse, believing him to have knowledge of the present whereabouts of your exiled master, He Who Must Not Be Named -"
"Father, I didn't!" shrieked the boy in chains below. "I didn't, I swear it, Father, don't send me back to the Dementors -"
"You are further accused," bellowed Mr Crouch, "of using the Cruciatus Curse on Frank Longbottom's wife, when he would not give you information. You planned to restore He Who Must Not Be Named to power, and to resume the lives of violence you presumably led when he was strong."
(Goblet of Fire, Chapter 30, The Pensieve).
Why on earth would Frank Longbottom be expected to know the current whereabouts of Lord Voldemort? Presumably, if he had have known then he would've tracked Voldemort down himself with a group of Aurors. It seems bizarre that Longbottom should be singled out as this great source of potential intel. Why was Longbottom picked over and above everybody else? Did it have anything to do with the prophecy?
I guess the overarching question is: did Frank Longbottom actually have any information that was any use to them (because if he did that would explain why they went after him)? Did he lose his sanity as a hero, denying them a crucial piece of information that would actually help them? Or did he go down because of he genuinely couldn't tell them what they were trying to find out?