LOS ANGELES (Reuters) – Family members of four passengers who died with singer Jenni Rivera when her plane crashed in northern Mexico last year sued her corporation and the current and former owners of the aging Learjet they called a "bucket of bolts" on Thursday. The plaintiffs alleged in their Los Angeles Superior Court lawsuit that the 43-year-old aircraft was unsafe and being flown by two unqualified pilots when it nosedived from 28,000 feet into mountainous terrain. "We are trying to unravel the mystery of how this came to be. …