Firefighters suspect an arsonist began a small duplex fire Monday in Modesto's airport neighborhood that caused about $40,000 in damage, officials said. No one was injured. It appeared to have been intentionally set, said Battalion Chief Mark Johansen of the Modesto Fire Department. Police detained a woman for questioning but later released her. The investigation will continue, officials said. The single-alarm fire, which broke out about 11 a.m. in the 700 block of South Conejo Avenue, was under control within 10 minutes, Johansen said.