Police are working to identify the Manchester City fans who set off flares inside St James' Park during the 3-1 win against Newcastle on Saturday.
One of the flares was thrown from the section containing travelling spectators into another containing home fans below.
A Northumbria Police statement said: "A flare was thrown from the section containing Manchester City fans into a section of Newcastle United fans, no one was injured and it was quickly extinguished.
"A second flare was set off in the section containing Manchester City fans. Officers at the scene carried out extensive enquiries, including checking CCTV and speaking with available witnesses. However, the person or people responsible are yet to be identified.
"We take incidents such as this extremely seriously and enquiries are still ongoing. Officers continue to work with colleagues at Greater Manchester Police.''
The investigation is one of two launched by the Northumbria force following the game. Police also received a complaint about City substitute Aleksandar Kolarov who is alleged to have become involved in a lengthy exchange with fans holding an Albania flag as he warmed up on the sidelines.
Information from the Press Association was used in this report.