FORT WORTH, Texas — The cause of an explosion, believed to be linked to a natural gas leak, that caused massive damage to the Sandman Signature Hotel in downtown Fort Worth was still under investigation Tuesday. Multiple people, all believed to be adults, were injured in the blast Monday afternoon, with 21 hospitalized or treated on scene, according to the fire department and MedStar. A MedStar spokesperson said the count still stood at 21 on Tuesday. Fourteen were transported to local hospitals, with one in critical condition and four in serious condition. The most severely injured person, a …