Cesar Florez is often hesitant to answer his phone because there might be another death threat at the end of the line. Sometimes the threat comes in a phone call, other times in a text message or an email. In April, flyers were posted in the restroom stalls at Florez’s workplace, declaring him and his colleagues “permanent military targets.”

Until last month, Florez served as a local president of Sintramienergetica, a labor union in Colombia that represents the employees of Drummond Company, a U.S.-based coal-mining firm, in a country known for some of the world’s most severe violence against union leaders. Florez has been a Drummond employee for 17 years and active in the union for the last 14. Most recently, he worked as a marine operations technician in Drummond’s port near Santa Marta, where its coal is shipped out on barges.

But his position as a union leader has also meant he’s attracted a significant number of threats, including attempts on his life, which happen to spike around labor disputes, he said. In July 2013 the union went on strike, calling for a pay raise and to move from an hourly wage to a salary, among other demands. For 53 days the strike wore on amid tense negotiations, while the threats that Florez and his colleagues received only accelerated.

“They said if we didn’t lift the strike we’d be a target,” Florez said, describing some of the phone calls he received. “They said they already knew where my family was.”

Many of the written threats that Florez received bear the watermark of Los Rastrojos Comandos Urbanos, an active paramilitary group with ties to drug trafficking.

The Center for Public Integrity made numerous attempts to reach Drummond for comment on allegations that it has used the group to try to intimidate Sintramienergetica leaders like Florez; a spokesman said he could not respond to any questions on the matter. In a recent statement, the company’s lawyers asserted, “Drummond has never paid or otherwise assisted any illegal group in Colombia, whether paramilitary or guerilla [sic].”

Nonetheless, Drummond has been named in several lawsuits alleging financial ties to paramilitary groups since the mid-1990s.

Drummond — a closely held company based in Birmingham, Alabama, with revenues that reached $3 billion last year—has helped Colombia become the world’s fourth-largest coal exporter. Heman Drummond started the business in 1935 on the backs of mules that were used to haul loads of coal from its mines in Alabama. Under the leadership of his son, Garry, the company expanded, securing a contract to extract coal in La Loma, in the Cesar Department of Northeast Colombia in the late 1980s.

While its Colombian operations quickly became a significant revenue stream for the company, security issues and labor disputes have always been substantial obstacles for Drummond’s business. And, according to its workers, intimidation has become routine in a country where trade union leaders are often viewed as subversives.