How safe are shore excursions for cruise line passengers?

That’s a question that resurfaces after highly publicized accidents such as December’s fatal bus crash in eastern Mexico that killed passengers of two ships owned by Royal Caribbean Cruises. Eleven foreign tourists and a Mexican tour guide died.

Cruise lines say they work as hard as they can to investigate companies that provide the excursions passengers can purchase when they book their cruises or after they board.

But a maritime lawyer who authors a blog critical of the cruise industry says cruise lines work with too many excursion companies to possibly be able to conduct thorough background checks.

Officials with Miami-based Royal Caribbean did not immediately respond to questions seeking details about how the cruise line selects excursion companies and investigate their safety records, and whether the companies are regulated within destination countries.

Carnival Cruise Lines spokeswoman Jennifer De La Cruz provided this response: “Before accepting any shore excursion operator into our program, the operator must meet a variety of established criteria to demonstrate professionalism, safety standards, insurance, and overall ability to deliver a quality experience to our guests.

“Guest feedback on shore excursions is constantly reviewed and tours are periodically assessed by company personnel in an effort to ensure standards are continuing to be met.”

With cruise lines docking in dozens of countries around the world each day, passengers who want to book excursions have little choice but to take the cruise lines at their word.

Jim Walker, a Miami attorney specializing in maritime law, advises consumers to debark with caution. In an interview last month, Walker said cruise lines don’t have the personnel to conduct thorough background checks on all of the excursion companies offered to its passengers at all of their ships’ ports of call.

Although they are obligated by U.S. maritime law to conduct background checks on excursion companies and to warn guests of dangers on shore, they often rely on endorsements from local tourism authorities or even from other cruise lines that also contract with the companies, Walker said.

“They’re just relying on word of mouth. They don’t have the manpower or interest in performing real vetting,” he said.

The fatal crash in Mexico was the latest in a series of bus crashes by shore excursion companies.

In January 2016, a tour bus collided with a truck in Falmouth, Jamaica, killing a passenger of Royal Caribbean’s Independence of the Seas and injuring more than a dozen others on a cruise line-offered excursion. A lawsuit filed against Royal Caribbean alleged the bus driver was driving erratically, speeding and frequently changing lanes.

In November 2016, a bus carrying 10 passengers from the Carnival-owned P&O Azura crashed in Dominica, killing a British passenger and injuring nine others.

In 2010, a tour bus carrying passengers to an excursion sold by Princess Cruises crashed in Tortola, British Virgin Islands, killing one and seriously injuring two others.

Accidents that occurred after the buses were parked included a crash of a sightseeing plane that killed the pilot and eight passengers of Holland America’s MS Westerdam during a cruise to Alaska in June 2015. An investigation by the National Transportation Safety Board determined the sightseeing company had created a culture in which pilots felt compelled to risk flying in low-visibility conditions, the Seattle Times reported.

Still, a recent post on cruise enthusiast website cruisecritic.com advises that excursions booked through cruise lines are a far safer bet than excursions offered by independent operators soliciting on the docks.

“When your cruise line suggests you avoid independent tour operators, it’s not because they’re trying to earn an extra buck; it’s because they know for certain their operators are legit,” the post said. “That’s not to say accidents don’t happen on cruise-sponsored excursions. But you at least gain more peace of mind knowing an official authority did its due diligence on your behalf.”