A senior officer in Hong Kong police's National Security Department has been placed on leave after he was caught in a raid on an unlicensed massage parlour, say local media reports.
Reports have named him as Director of National Security Frederic Choi.
According to news site SCMP, visiting an unlicensed parlour is not an offence but could "damage the force's image".
This is the first big scandal to hit the unit after it was set up to enforce the city's national security law.
Thousands in Hong Kong protested last June when the national security law was implemented.
The law criminalises secession, subversion and collusion with foreign forces with the maximum sentence life in prison. However, Beijing has said it will target "sedition" and bring stability.
Since the law was enacted around 100 people - including prominent pro-democracy activists - have been arrested.
Hong Kong police told BBC Chinese in a statement that they were investigating "an allegation of misconduct by a police officer", adding that the person involved had been placed on leave while the investigation continues.
"The police attach great importance to the conduct of police officers... [we] will conduct a fair investigation," it added, though it declined to give further details on the incident or name the person involved.
A senior police insider had told the South China Morning Post that while visiting or "buying service at an unlicensed massage parlour might not constitute an offence that could result in an arrest, Choi could face a disciplinary hearing as his conduct affects the force's image and damages its reputation."
Reports say Mr Choi became a police inspector in 1995, and was made senior assistant commissioner and director of the National Security Department last July.
In February, he was one of seven officers honoured at a ceremony by Hong Kong chief executive Carrie Lam for their contributions in safeguarding national security.
Mr Choi had reportedly been a favourite to head the police force in the future, according to local reports.