Witness to Murder (English, Paperback, Franklin W. Dixon)

While trying to help Annie Shea, the pretty new girl in town, Joe accidentally runs down her old boyfriend, Phil. Joe’s in a tight spot, but Annie seems afraid to help. Then the Hardys learn that Phil is the prime suspect in a million-dollar diamond robbery and Annie may be involved.

But when Annie is kidnapped the brother detectives swing into action. They follow her to the liar of America’s most infamous gem thief, Cutter. Outnumbered, the Hardys take on the diamond man’s gang in an all-out effort to save Annie — and clear Joe of murder.
Source- Goodreads.com