After “an alarming increase in worker deaths” last year, the U.S. Labor Department is urging mobile phone providers to redouble their focus on safety at cell tower sites across the nation.

According to the department’s Occupational Safety and Health Administration (OSHA), 13 workers died at communication tower worksites in 2013 — more than in the previous two years combined. Four more were killed in the first five weeks of 2014, continuing what the agency called a “disturbing trend.”

“Tower worker deaths cannot be the price we pay for increased wireless communication,” said David Michaels, assistant secretary of labor at OSHA, in a statement.

During the last eight years, the number of U.S. smartphone subscribers has shot up by a factor of 44, from 3.5 million in 2005 to 156 million in 2013, according to comScore. However, the surging demand for better and faster cell phone service has come at a steep price.

A 2012 investigation by FRONTLINE and ProPublica found that between 2003 and 2011, nearly 100 climbers were killed on radio, TV and cell towers, a rate that was about 10 times the average for construction workers. Fifty of those deaths were at cell sites.

The investigation, Cell Tower Deaths, found that a complex web of subcontracting has allowed the nation’s major phone carriers to avoid scrutiny when accidents happen. Deaths were found to result from climbers lacking the proper safety gear or because they were sent hundreds of feet into the air with little training. To meet urgent demands from carriers or contractors, climbers sometimes worked overnight or in hazardous weather conditions.

Findings from OSHA suggest continued cause for alarm across the entire industry. In a letter sent to mobile firms this week, the agency voiced concern “about the possibility of future incidents, especially when the hazardous work is done by employees of subcontractors.” OSHA said that a “high proportion” of deaths were cause by “a lack of fall protection,” and that over the last several months, tower workers have been injured or killed by falling objects, equipment failure, and collapsing towers.

In a somber assessment, the agency concluded, “Every single one of these tragedies was preventable.”

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