The letter, addressed to Criminal Investigation Department (CID) at the Police Headquarters, was discovered by staffers at around 2pm.

After reporting the incident, officers from the Explosive Ordnance Disposal Bureau and firefighters soon arrived at the scene to take care of the letter. They left at about 3pm.

Primary investigation suggested that the letter was delivered there by a mailman around 10am in the morning.

On Monday, a threat letter containing a razor blade was sent to Chief Executive Carrie Lam Cheng Yuet-ngor.

Sources said the letter to Lam contained political demands, including releasing protesters of the 2019 anti-fugitive bill movement.

It is not the first time the Police Headquarters has received such a threat letter.

On May 13, a staffer there discovered a letter addressed to then-Police Commissioner Chris Tang Ping-keung. Officers found that the paper may have been dipped into water with an unknown powder dissolved in it. Further examination revealed that the powder was roasted soy flour.

A letter containing corn flour was sent to the Police Headquarters in April last year as well.