Homicide detectives for San Diego County Sheriff's Department Sunday were investigating the death of a 17-year-old El Cajon girl whose body was found partially submerged in a bathtub at a home where she was employed to do housework. The body of Christine Lorelle Pain, a junior at Granite Hills High School, was discovered Saturday at the Sunview Drive home of Leon and Sue Martelli, said Charles Kelley, a deputy coroner. He said there was evidence of both strangulation and drowning. The fully-clothed body was discovered by Sue Martelli when she arrived home about 4:40 p.m., said Sheriff's Sgt. D.Q. Martin. Kelley said that tests to determine whether Pain had been sexually assaulted would be performed today. The Sheriff's Department declined to supply further details.