Although Wolfe's investigation begins unpromisingly, he becomes convinced that his suspicions are correct when the police receive an anonymous tip revealing Laidlaw's secret. Although the police are skeptical due to the tip's anonymous nature, it suggests to Wolfe that someone else knows Laidlaw's secret and has become agitated by the ongoing investigation.
Despite a slow start, Wolfe becomes convinced of his suspicions when the police receive an anonymous tip pointing to Laidlaw's secret.  The police are hesitant due to the anonymous nature of the tip, but it suggests that someone else knows Laidlaw's secret and is disturbed by the investigation.