How would you identify the source for the causative agent. how would you determine if it was a common source?

In 1976, an outbreak of a respiratory disease occured at a hotel in Philadelphia where a convention of the American Legion was being held. The previously unrecognized disease was named Legionnaire's diseas. ume you were the epidemiologist involved in this. How would you identify the source and/or reservoir for the causative agent, and how would you determine if it was a common source or propagated epidemic?