LONDON, ENGLAND - AUGUST 06: A London Underground employee is glimpsed at a deserted Waterloo Tube Station on August 6, 2015 in London, England. London Underground workers are staging another 24 hour strike after management and unions failed to reach agreement over new night time services. (Photo by Peter Macdiarmid/Getty Images)

London’s Metropolitan Police says a fire on the London subway has been declared a “terrorist incident.” The force says counterterrorism officers are leading the investigation into the incident at Parsons Green station. Passengers reported an explosion in a carriage of the train shortly after 8 a.m., during the morning rush hour. Several people appeared to have burn injuries. Police say it’s “too early to confirm the cause of the fire, which will be subject to the investigation that is now underway by the Met’s Counter Terrorism Command.” Health officials say the number of people injured in the London subway bombing has risen to 22. Eighteen were taken to hospitals by ambulance, and four more presented themselves at hospitals. None of the injuries is thought to be serious or life-threatening.The National Health Service says the patients are being treated at four London hospitals and clinics.

British police say no one has been arrested in connection with a bombing on a London subway train, but hundreds of detectives are at work trying to hunt down the perpetrator or perpetrators. The Metropolitan Police force says police “are making fast-time inquiries to establish who was responsible and are working closely with the security services.” Counterterrorism policing chief Mark Rowley says hundreds of detectives are looking at surveillance camera footage, carrying out forensic work and speaking to witnesses.

U.S. President Donald Trump is calling a fire at a London subway station another attack “by a loser terrorist” and suggesting police there may have missed an opportunity to prevent it. He also is suggesting that the government cut off internet access to extremist groups. Trump tweeted Friday: “Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!” He later added: “Loser terrorists must be dealt with in a much tougher manner. The internet is their main recruitment tool which we must cut off & use better!”

German and French leaders say the bomb attack on a London subway train only strengthens their determination to increase international cooperation in fighting terrorism. Chancellor Angela Merkel said after meeting with French Prime Minister Edouard Philippe in Berlin on Friday: “Our thoughts are of course with the wounded, our thoughts are with the British population.” Philippe said the London bomb and an attempt early Friday by a knife-wielding assailant to attack a soldier at a Paris subway interchange “show how much we collectively, in France, Britain and also in Germany, face a major threat.” He added “we must find answers at national level and all together — to give our fellow citizens the greatest possible security,” including intelligence cooperation.