The deaths of Trent Vigus and at least nine other oil-field workers over the past five years had haunting similarities. Each worker was doing a job that involved climbing on top of a catwalk strung between rows of storage tanks and opening a hatch. There were no known witnesses to any of the men’s deaths. Their bodies were all found lying on top of or near the tanks. Medical examiners generally attributed the workers’ deaths primarily or entirely to natural causes, often heart failure.

But in the past few months, there has been a shift. Though still unsure of the exact cause of the deaths, government agencies and some industry-safety executives are now acknowledging a pattern and are focusing on the possible role played in the deaths by hydrocarbon chemicals, which can lead to quick asphyxiation or heart failure when inhaled in large quantities.

In the meantime, federal agencies and industry-safety groups are planning to send out a joint alert to the oil industry as early as this week, warning of the potential for imminent danger from inhaling hydrocarbons, according to several people involved in the effort. Much of the industry remains ignorant of the possible risks, they say.

David Miller, the director of the American Petroleum Institute’s standards board, said the industry may also address the issue while working on new recommendations involving tank storage. “It is an acknowledgment from our perspective that one accident is too many,” he said.

According to some industry-safety and government officials. The industry has been ignoring warning signs for years and has been resistant to implementing some steps that would reduce or eliminate the risk to workers.

“I was trying to get workers into respirators and all kinds of things and running an uphill battle,” said a former industrial hygienist for a large oil company who said he had noticed dangerously high hydrocarbon levels in some of his testing as far back as 2009. “They say, ‘Everyone does it this way.’ But that doesn’t make it any less right or wrong.”

Some industry officials said that companies hadn’t realized there might be a problem until the pattern of deaths began to emerge, but they now acknowledge the situation needs to be studied further.

The documented deaths date back to 2010, with six last year. Three were in North Dakota, three in Colorado, one in Texas, one in Oklahoma and one in Montana, according to the Centers for Disease Control, which first highlighted the pattern in May 2014. This year there was at least one death, in North Dakota. It’s still under investigation and may fit the pattern, federal officials said.

Four workers were engaged in what’s called tank gauging, where they measure the level of oil in tanks coming out of wells or the level of some byproducts that come up after the fracking process is completed. Five others were assigned to take samples of oil for further testing.

These jobs are often done alone in remote areas and require opening the hatch and standing above it. When that happens, chemicals that have vaporized spurt out in a dense plume that is often invisible, safety experts say.

It’s unclear why these types of deaths were noticed only recently, and the cause is still under study. Unlike many previous oilfield deaths involving toxic fumes, the culprit doesn’t appear to be hydrogen sulfide, which has long been well-known in the industry as a hazard. Some experts have focused on the unusually high levels of certain hydrocarbons, including benzene, in the type of crude that is now common in the U.S.

In one small sample, the U.S. Occupational Safety and Health Administration found that some chemicals that came out of the tanks close to where some workers stood exceeded levels that have the potential to cause imminent death or permanent health effects, according to a study the agency presented in December.

Some industry and government experts say they believe the danger may be exacerbated in part by a recent environmental rule designed to protect public health.

In 2012 the Environmental Protection Agency enacted a rule that new oil-field tanks would have to capture hydrocarbons coming out of well sites rather than vent them into the atmosphere. As a result of the requirement, the same dangerous chemicals that had been emitted regularly were now more likely to build up pressure inside the newer tanks, industry and government scientists say.

A spokeswoman for the EPA said its storage-tank requirements shouldn’t add to hazards if they are properly designed and operated.

In July 2010, Mr. Vigus, a 30-year-old worker on a Montana well site, was found slumped over on a catwalk near a storage tank he was checking. The county’s pathologist determined he had died of heart failure.

Mr. Vigus’s mother, Terri Vigus, said the report had always left her uneasy. “I just never, ever felt that that was what happened,” Ms. Vigus said. “I just kept going to how healthy he was.”

OSHA safety inspectors closed their case with no findings around two months later, when they received the medical report.

Some industry experts say the industry knew the plumes could unleash potentially dangerous vapors and should have been monitoring the chemical levels all along. And, they say, companies could implement safety fixes that would reduce or remove hazards. One option is to use automated or remote methods to read tank levels. That is done regularly elsewhere, including in Canada.

“There’s no question in my mind it was absolutely known” that there were dangerously noxious fumes coming from the tanks, said Dennis Schmitz, a safety consultant for oil companies in North Dakota. “You are absolutely required to evaluate that hazard before you put that employee up there.”

“Every hazard should be engineered out,” added Mr. Schmitz, who acknowledged that fixes would add some cost.

In a case related to one of the deaths, an environmental engineer for Marathon Oil noticed in his emissions studies that his company was using pipes that were too narrow to accommodate the pressure of the gas coming through them, creating too much gas buildup in the tanks, according to a sworn statement he gave in a later lawsuit after the worker, Dustin Bergsing, died on a Marathon Oil site.

The engineer said he asked the company to redesign some of the piping systems to create more of a constant flow but was ignored.

Lee Warren, a spokeswoman for Marathon Oil, said in an email that the company considered that statement to be “grossly inaccurate and wholly without merit.” Marathon settled the case; it did not admit liability. “We take seriously the responsibility to properly educate employees about potential hazards that could be encountered in their work,” she said.

Kenny Jordan, the executive director of the Association of Energy Service Companies, said the first priority for the industry is to raise more awareness among workers about the dangers. “We’re trying to take a proactive approach with what we’re doing and trying to get the notice out to workers in the field,” Mr. Jordan said.

Federal officials say they have limited power to force the industry to do more. Oil and gas sites are exempted from many OSHA rules, including specifications on how to handle potential benzene exposure.

At a recent online presentation designed to educate North Dakota oil workers about hydrocarbon dangers, a panel made up of federal and industry safety officials as well as some oil companies showed workers an infrared video image of a giant plume of chemicals coming out of the top of a tank that they said was otherwise invisible.

The officials told workers they should stand upwind of the chemicals when they open the tank hatches.

Workers should also hold a tool called a “four-gas monitor” over the top of the tank, the officials told workers, though all workers aren’t currently provided with the tool.

But afterwards several experts, including some panel members, said those measures won’t solve the problem. Said Timothy Hicks, an oil-field safety consultant: “Wind is not a reliable exposure control.”

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