The Ministry was under great pressure to catch those who had done it. Unfortunately, the Longbottoms’ evidence was — given their condition — none too reliable.

"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."