SAN DIEGO -- State officials announced $67,500 in fines Wednesday against a Sorrento Mesa contracting company in connection with a natural-gas explosion that injured 13 workers at a Marina-area hotel construction site last spring.

The California Occupational Safety and Health Administration found that Sherwood Mechanical Inc. was primarily responsible for the blast, which tore a gaping hole in the unfinished San Diego Bayfront Hilton six months ago.

Three of the injured workers suffered critical burns requiring lengthy hospitalization and extensive skin-graft procedures.

Cal-OSHA levied four fines categorized as "serious" against Sherwood Mechanical, concluding that the firm failed to properly train its employees to prevent and test for workplace natural-gas leaks.

Representatives of the plumbing contractor did not immediately return calls for comment.

The explosion, which occurred just after 2 p.m. May 19, was centered in a fifth-floor utility room at the 30-story structure being built near Petco Park.

It blew out walls and windows on the unfinished high-rise, sent large chunks of debris flying hundreds of feet, shook buildings for blocks around and was loudly audible for miles. The damage at the construction project was estimated at $5 million.

The state investigation into the mishap also led to citations against six other contractors working on the building for various "general regulatory violations" involving such issues as record-keeping and posting of rules, Cal-OSHA spokeswoman Erika Monterroza said.