Injuries at London's Borough Market after unknown substance delivered

Three people were being treated for minor injuries on Thursday after an envelope containing a substance was sent to a business at London's Borough Market, police say.

Police did not say which business was sent the letter, but a CNN journalist there saw a police cordon placed around the Feng Sushi Japanese restaurant.

London Metropolitan Police received reports of the incident and were called to the area -- an enclave of bars and restaurants near London Bridge -- after 1:30 p.m. (08:30 a.m. ET).

Images on social media showed an ambulance and firefighters outside a restaurant responding to the incident earlier.

The property was evacuated as a precaution, police said, and a staff member who tried to enter the restaurant was seen being turned away by police.

Police are not treating the incident as terror-related and said that the substance has been identified as "naturally occurring."

While it is unknown what substance was in the envelope, there is growing awareness of an increase in acid attacks in the city.

Borough Market was targeted in a terror attack in June, when three men went on a stabbing spree after ramming a vehicle into a crowd on London Bridge. Seven people were killed and 48 were wounded in the attack.