Two people are still in hospital after the ceiling of a theater in London partially collapsed on Thursday evening, local authorities said on Friday. They are not in life-threatening conditions, authorities said. The London Fire Brigade has revised down the number of casualties to 76 on Friday from 88 as previously reported, among whom seven had been described as in serious condition. To figure out cause of the accident, the Westminster City Council has carried out a structural assessment into the 100-year-old Apollo Theater, which opened in 1901 and is a Grade II-listed building. They are also investigating the influence of weather on the theater. When the accident happened at 8:15 p.m. on Thursday, London was swept by heavy rain accompanied with thunder and lightening. A BBC report said that some people felt water dripping through cracks in the ceiling before part of it came down suddenly. Police remained at the scene overnight to tell people to stay away from the theater, so that the investigation would not be affected. London Mayor Boris Johnson praised the response of emergency services. "The emergency response, in particularly confused and trying circumstances, was rapid and the co-ordination between the Metropolitan Police, the London Ambulance Service and the London Fire Brigade was exemplary," he said. "I would stress that although it is too early to say what caused this collapse," he said, "as a precaution, further checks (for West End's historic theaters) have already started and will continue throughout the day." The West End in London is a famed for its mainstream professional theatres. Like the Broadway in New York, they are considered to represent the highest level of commercial theaters in the English-speaking world. Andrew Cooke, Chief Operating Officer at London and Partners, the Mayor's promotional organization for London, said incidents like this were "extremely rare". "Every year millions of tourists enjoy the experience of London's renowned theatrical and musical performances and last year alone 14 million people attended the theatre in the capital," he said. "We welcome the fact that London theaters have said they will cooperate with authorities to reassure the public that venues are safe and we are confident that they will continue to delight and entertain millions of visitors in the future," he added. The accident happened 40 minutes after the popular play The Curious Incident of the Dog in the Night-Time started. The play, originally performed at the National Theater, was moved to Apollo in March. A spokesperson from the Society of London Theatre told press in a statement that performances have been canceled on Friday and Saturday. "Customers should contact their point of purchase to arrange exchanges or refunds," said the statement. "The National Theatre and Apollo Theatre will advertise details regarding the future status of performances once this is known."