Sidney P. Kilmartin knew when he sent potassium cyanide to a depressed man in the United Kingdom that the man intended to use it to kill himself.

According to a summary of the investigation released Thursday by Humberside Police in England, Kilmartin and Andrew Denton of Kingston-upon-Hull exchanged emails in which Kilmartin offered to sell Denton cyanide and gave him advice on how to use it.

The first batch of cyanide that Kilmartin sent in 2012 failed to kill Denton and he complained about its quality. Kilmartin then allegedly mailed him another batch. Denton, 49, was found dead on Dec. 31, 2012, with 17 milligrams of cyanide per liter in his blood – more than three times the lethal dose of 5 milligrams per liter, according to Humberside Police.

While the British authorities provided more details about the case than federal authorities in Maine, who arrested Kilmartin at a gym in Windham on Wednesday, no one on either side of the Atlantic is saying how Kilmartin was able to obtain the poisonous chemical or how he and Denton found each other despite living nearly 3,100 miles apart.

“Neither is information we are in a position to release,” said Matthew Wright, a spokesman for the Humberside Police, a district force that oversees a section of northeastern England.

It’s not known whether Kilmartin sent cyanide to anyone else.

Authorities allege Kilmartin was able to obtain cyanide – and deal it internationally – despite a history of mental illness and drug addiction.

He had been involuntarily committed to Riverside Psychiatric Center in Augusta until less than a year and a half before he is accused of mailing the first batch of cyanide overseas on Nov. 16, 2012. He is accused of mailing the fatal dose on Dec, 11, 2012, while living in an independent apartment with psychiatric supervision, according to state and federal court records.

Dr. Karen Simone, director of the Northern New England Poison Center at Maine Medical Center, said that while it’s possible to get cyanide through the Internet, she believes it is still rare and difficult to locate.

“In cases that I’m aware of, oftentimes it’s a person who has access to a lab or an industrial site where it’s available,” Simone said. “The fact that we have very few cyanide deaths that do occur would indicate that it’s not easy to get. But with today’s Web access, I’m sure you could get anything you want if you tried hard enough.”

Kilmartin, 52, of Windham, was arrested Wednesday after being indicted by a federal grand jury on charges of mailing injurious articles and mailing injurious articles leading to death. The latter, more serious charge is potentially punishable by the death penalty, but prosecutors in Kilmartin’s case said they are not pursuing that option. He faces a maximum sentence of life in prison.

Kilmartin pleaded not guilty to the charges Wednesday afternoon in U.S. District Court in Bangor. He is being held at the Somerset County Jail pending a detention hearing scheduled for Nov. 14 at 1:30 p.m.

Federal court filings in both Portland and Bangor contain few details of the government’s case against Kilmartin.

The British police who investigated Denton’s death said they began looking into where Denton obtained the cyanide he used to kill himself as soon as a coroner determined he had died by poisoning.

“They identified a series of online exchanges between Mr. Denton and a person from Maine in the United States of America using a pseudonym after analyzing a mobile phone, laptop and other electrical items that had been seized from Mr. Denton’s home at the time of his death,” Wright, the Humberside Police spokesman, wrote in the investigative summary.

British authorities then contacted American officials and worked with the U.S. Postal Inspection Service to trace the package and emails back to Kilmartin, Wright said.

Postal Inspector Michael Desrosiers flew to England in June to interview witnesses. Authorities had the remnants of cyanide found near Denton’s body taken to a U.S. Air Force base to be returned to Maine, Wright said.

Kingston-upon-Hull, often abbreviated as Hull, is a city of more than 250,000 about 200 miles north of London.

Denton had spent years struggling with depression before his death and had made multiple suicide attempts before taking the fatal dose of cyanide, according to an article last year in the Hull Daily Mail.

“I am delighted that we have been able to support the U.S. Postal Inspection Service with their case, which has led to a man being arrested for enabling Andrew Denton to commit suicide. I know that Andrew’s family (is) grateful, and that is what our team strives for,” said Humberside Police Detective Sgt. Ian Dobson, who led the team investigating Denton’s death.

Potassium cyanide is a highly toxic, colorless salt, similar in appearance to sugar, according to the U.S. Centers for Disease Control and Prevention. Used commercially for fumigation, electroplating and extracting gold and silver from ores, it is usually shipped as capsules, tablets or pellets. It releases highly toxic hydrogen cyanide gas, which interferes with the body’s ability to use oxygen. Swallowing the chemical causes nausea, vomiting, abdominal pain and irritation or corrosion of the lining of the esophagus and stomach. Ingestion can quickly become fatal.

State court records in Augusta document Kilmartin’s history of institutional care for mental illness.

Kilmartin was committed to Riverview Psychiatric Center from Sept. 9, 2008, until he was released under supervision on May 19, 2011. He was living on his own and seemingly on a positive path at the time he allegedly sent the cyanide to Denton, court records state.

Kilmartin had been committed to Riverview after being found not criminally responsible by reason of insanity for smashing a radio into the head of an 86-year-old man in October 2007. Kilmartin was living upstairs from the man on Bailey Avenue in Portland at the time.

After his stay at the psychiatric hospital, he transitioned first to a supervised living apartment, then to an independent apartment in Manchester, and from working part time at Sam’s Club in Augusta to working full time rehabilitating houses in September 2012, according to an institutional report contained in court filings in Kennebec County Superior Court in Augusta.

Despite the positive reports, Kilmartin was on the verge of a drug relapse, according to records filed with the court this year.

Before Kilmartin’s commitment at Riverview, he had been diagnosed with bipolar disorder and had a history of cocaine abuse. By February 2013, after he allegedly mailed the cyanide to England, he tested positive for cocaine and alcohol during toxicology screens conducted by the outpatient services program at Riverview, court records state.

He was briefly committed to Riverview again, released, but then tested positive for crack cocaine by May 2013. After several months in a long-term residential program for substance abuse and mental health treatment, he was released again on Oct. 29, 2013, and allowed to live in his home in Windham, according to court records.

It is unclear whether Kilmartin’s latest arrest will prompt authorities to have him recommitted to Riverview.

During Kilmartin’s initial court appearance Wednesday, his attorney, Jeffrey Silverstein, asked to reserve the right to alter his pleas to not guilty by reason of insanity at a later date.