The exotic and toxic sedative named carfentanil has been linked in recent weeks to fatal overdoses in Illinois, Colorado, Wisconsin and Minnesota.

A substance used to tranquilize elephants that is 100 times more potent than the drug that killed Prince is presenting a new and difficult challenge in the nationwide opioid epidemic.

"We have never seen death like we do now," said Tom Synan, head of the Hamilton County Heroin Coalition in Ohio, which was among the first spots to discover a string of carfentanil deaths during a week in which the county's overdoses more than doubled.

On Monday, a Virginia man pleaded guilty in a drug distribution case after selling $100 of carfentanil-laced heroin to a 21-year-old found dead by her mother on the bathroom floor of their Fairfax County home.

Three cases in Anne Arundel and Frederick counties this month mark the first carfentanil-related fatalities in Maryland, alarming local health and law enforcement officials already in a state of emergency combating the opioid crisis.

The difficult-to-detect substance is so powerful that an amount equivalent to a few grains of salt can be deadly. It requires more aggressive treatment than a typical opiate overdose to reverse. First responders are getting burned out answering back-to-back overdose calls rising because of carfentanil and other synthetic opioids, and they worry about falling ill after exposure while answering calls.

The drug is so new that some medical examiners don't have the tools to detect it in autopsies.

"It shows how callous these drug dealers are," Synan said. "It has no human use whatsoever, and they're putting it out on the street and wreaking havoc."

Hamilton County, which includes Cincinnati and nearly 50 law enforcement agencies, experienced an average of 50 to 70 reported overdoses a week in early 2016 and four or five deaths, Synan said. One month after law enforcement learned carfentanil had hit the county, overdoses skyrocketed, with about 175 to 200 calls in a single week in August. Four of those users died.

Often, people don't know drugs they've purchased have been laced with an elephant sedative that is 10,000 times as powerful as morphine, leaving their families devastated.

"I'm desperate to get some stuff but I don't know anyone that I could buy from," 21-year-old Kristina Lutz texted a drug dealer Aug. 29, according to Fairfax County court documents. "Any chance you'd be able to help me out haha."

Hours later, Lutz stuck a needle in her arm with carfentanil-laced heroin and died at her family's home in Fairfax Station.

Her parents described Lutz as suffering from anxiety and depression while growing up, but they said she seemed happy recently and was looking forward to running in a race. Her parents said she'd had a recent setback when her boss at a dog-boarding and day-care site died of cancer.

"We tried to bring her to a safe environment and give her a good life," said Kristina's father, Jeffrey Lutz, who with his wife adopted their daughter from a Russian orphanage when she was 16 months old. "Ultimately, because of the evils of these deadly drugs and somebody willing to sell it to her, we couldn't keep her safe."

Melvin Patterson, a spokesman for the Drug Enforcement Administration, said that carfentanil cases emerged in the early 2000s, but a U.S. resurgence started in mid-2016. In quick succession, spikes ravaged communities in Ohio, West Virginia, Kentucky and New Hampshire, prompting the agency to issue a warning in September about the "crazy dangerous" drug.

In its intended use, a little carfentanil goes a long way.

All of the zookeepers and vets in the United States combined need only about 18 grams of carfentanil a year, about the weight of 18 artificial sweetener packets, Patterson said. "If they don't need very much of it to use on an annual basis to tranquilize big, large animals, then we humans don't stand up at all," Patterson said.

It's unclear why there has been a recent resurgence in the drug, but typically, "everything is about money," Patterson said. A small amount of carfentanil can infuse mass quantities of heroin, increasing profits.

About two milligrams of fentanyl — about what comes out with a single jiggle of a salt shaker — is considered lethal. Carfentanil is 100 times as strong. Carfentanil and fentanyl, the substance found in music legend Prince's body when he died last year, has been manufactured mostly in China, Patterson said. But DEA officials hope a recent production ban in China will curb its prevalence.

"Some people are looking for it because it is a higher high," said Lt. Ryan Frashure, a police spokesman for Anne Arundel, where fatal carfentanil overdoses occurred April 1 and April 12. "And then you have those who don't know and take their normal dose and end up overdosing from their normal amount."

The resurgence of the drug challenges first responders.

Because the drug can be absorbed through the skin, police and lab workers need to take strict precautions against exposure, even from a puff escaping a bag being resealed, said Scott Maye, the chemistry program manager for the Virginia Department of Forensic Science.

"Dogs can get a whiff of it, and it can be fatal," Maye said, adding that lab workers are offered masks and gloves and that testing equipment is stored in locked safes.

Many police departments have stopped testing for heroin at crime scenes because the possible presence of carfentanil makes it too risky.

The volume of carfentanil and fentanyl overdose calls is also overwhelming police and first responders, who are being diverted from other calls.

"We on the front lines are struggling every day to keep people alive," said Synan, president of the Hamilton County Association of Chiefs of Police.

In most cases, it can take months before authorities discover someone died of carfentanil.

Carfentanil is next to impossible to detect in toxicology tests, making it hard for authorities to identify quickly changing formulas and warn the public, said Rosie Hobron, Virginia's forensic epidemiologist for the Office of the Chief Medical Examiner.

"It puts us behind a curve," Hobron said.

Because of its stealthy nature, she said, Virginia technically has no confirmed carfentanil deaths. In Lutz's case, the drugs in her bathroom tested positive for the animal tranquilizer, something her family didn't learn until eight months later.

To fight the spread of carfentanil, police have been cracking down on drug dealers. Last week, a judge sentenced an Ohio man to 18 years in federal prison for selling drugs that led to 26 carfentanil overdoses last year, prosecutors said.

On Monday, the man who sold Lutz the drugs that killed her was convicted.

Lisa Lutz said she will never forget finding her daughter on the floor of her bathroom, not far from her bedroom, carefully decorated with photographs and a whiteboard marked with reminders for the future: Be positive; spend more time with family.

She struggled in school but rode horses and worked at the kennel. She was hoping for a career involving animals, possibly at a therapeutic riding center where she had volunteered.

Now her family is trying to figure out their own future. Lutz's parents think their daughter turned to drugs to treat depression and other mental-health issues and want to speak out against the drug scourge.

"She decided she was going to treat herself, treat the depression, and reached out to this guy," Lisa Lutz said. "If he hadn't been there willing and able to sell this deadly drug to someone who was emotionally frail at the time, I'm convinced she would be with us today."

The Washington Post's Justin Jouvenal contributed to this report.