Cyber crimes often involve complicated scenes. In this paper, we investigate unidentified crimes committed through anonymous communication networks. We developed a long Pseudo-Noise (PN) code based Direct Sequence Spread Spectrum (DSSS) flow marking technique for invisibly tracing suspect anonymous flows. By interfering with a sender's traffic and marginally varying its rate, an investigator can embed a secret spread spectrum signal into the sender's traffic. Each signal bit is modulated with a small segment of a long PN code. By tracing where the embedded signal goes, the investigator can trace the sender and receiver of the suspect flow despite the use of anonymous networks. Benefits of the Long PN code include its resistance to previous discovered detection approaches. We may also use the vast number of long PN code at different phases to conduct parallel tracback without worrying about the interference between codes. Using a combination of analytical modeling and experiments on Anonymizer, we demonstrate the effectiveness of the long PN code based DSSS watermarking technique.