They have summoned 14 people after the firing of a flare burned a woman to the bone of her left leg. Doctors have said she might have to undergo two surgeries.
The police decided Saturday that the 14 fans, all natives of Nam Dinh Province, one and a half hour south of Hanoi, will be investigated under the charges of "disrupting of public order" and "resisting persons on duty."
During the game between Hanoi FC and Nam Dinh FC, Nam Dinh supporters threw flares, and one of which hit the leg of To Huyen Anh, a Hanoi fan.
The game took place in Hanoi FC's Hang Day Stadium in Dong Da District. Nam Dinh fans also assaulted two police officers who had to be hospitalized.
Two more flares were thrown later, but luckily no one else was injured.
Police said the firing of the flares was being treated as a "serious" incident.
The clubs were fined VND85 million ($3,700) each by the Vietnam Football Federation.
As decided by the VFF, Hanoi FC will have to play the rest of their home matches this season behind closed doors.
They have two more matches at the Hang Day Stadium against Viettel FC on September 15 and Quang Nam on October 6.
Nam Dinh Club’s fans are banned from going to away games against Sanna Khanh Hoa on September 21 and Sai Gon FC on October 19.
Hanoi police said they had sent 150 officers to the field but "the number of fans was too high" and they used many tricks to hide the flares; and police "could not check each and every one."
Vietnam has banned the lighting of flares for all matches on safety considerations.
Hanoi FC admitted that failure to ensure property security had allowed fans to bring flares into the stadium. The club will take disciplinary action against chairman Nguyen Trong Chien, and will cover the treatment costs of the injured woman.