BANGOR — A Windham man with a history of mental illness pleaded not guilty Wednesday to federal charges that he mailed a poisonous chemical to a man in England who used it to kill himself.

Sidney P. Kilmartin, 52, is accused of sending potassium cyanide to a man identified in court records as Andrew Denton of Hull, England, who died on Dec. 11, 2012.

Kilmartin was arrested by federal agents Wednesday morning at a gym in Windham, according to a family member who requested anonymity to protect relatives. Kilmartin is under the care of the Maine Department of Health and Human Services, according to his attorney.

Kilmartin is accused of sending a package containing potassium cyanide to Denton in November 2012, leading to Denton’s death about a month later.

British authorities launched an investigation in conjunction with U.S. federal agents after learning that Denton, 49, had ordered cyanide on the Internet to kill himself following years of struggling with depression, according to a Hull newspaper report in 2013.

Authorities discovered remnants of potassium cyanide after Denton was found dead in his home. An autopsy confirmed that he died of cyanide poisoning, according to the Hull Daily Mail.

It is unclear how or where Kilmartin allegedly obtained the potassium cyanide or whether he knew what Denton planned to do with it. It’s also unknown whether he might have given cyanide to others.

Kilmartin was charged with mailing injurious articles and mailing injurious articles resulting in death. He could have faced the death penalty if convicted, but the U.S. Attorney’s Office announced it is not seeking the death penalty in this case. The maximum penalty faced by Kilmartin is life in prison.

“In all death-eligible cases, the attorney general has to file a notice of intent. The attorney general makes the final decision on all death-eligible cases,” said Assistant U.S. Attorney Donald Clark. “This defendant does not face the death penalty and would not face the death penalty.”

Kilmartin appeared confused after he was brought into U.S. District Court in Bangor to face the charges late Wednesday afternoon.

Before Kilmartin could enter his plea, his attorney, Jeffrey Silverstein, interrupted him.

“I would like to be able to come back and amend that plea with a not guilty by reason of insanity,” Silverstein told Magistrate Judge John Nivison. Kilmartin than pleaded not guilty.

Kilmartin, who appeared in blue gym pants and a light-gray sweatshirt, told the magistrate judge that he was aware of the charges against him and the potential penalties, but that he didn’t understand everything that was happening.

“It’s kind of overwhelming right now. I’m looking forward to speaking with (Silverstein) again,” Kilmartin said.

CASE INVOLVES UNCOMMON POISON

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.

Potassium cyanide 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. Exposure can quickly become fatal.

Hydrogen cyanide was the main ingredient in Zyclon B, a pesticide that the Nazis used to kill people in gas chambers. Cyanide also was used in the 1978 Jonestown massacre and the Chicago Tylenol murders in 1982.

“It’s not a common form of poisoning at all, but when it happens, it’s very bad,” said Dr. Karen Simone, director of the Northern New England Poison Center.

Since 2005, the center has reported four exposures to potassium cyanide in Maine. All were eye or skin exposures, three were work-related and all had minor physical effects. Two additional exposures were reported in New Hampshire and Vermont during the same period, including a case involving a suicide by ingestion. The other case involved skin exposure in which potassium cyanide wasn’t found responsible for the patient’s clinical effects.

Across the nation, poison centers reported 148 exposures to cyanides (excluding rodenticides) in 2012, most of them unintentional, according to the American Association of Poison Control Centers. Twenty-two were intentional suicide efforts and 12 were related to contamination, tampering or homicide. Seven exposures resulted in death.

As little as 200 milligrams – about the size of an aspirin tablet – can be fatal.

“If you swallow enough, you’re dead. It takes relatively small amounts,” said Dr. Frank Paloucek, a pharmacy professor and toxicologist at the College of Pharmacy at the University of Illinois at Chicago.

Cyanide can occur naturally and is produced by bacteria, fungi and algae. It is found in cigarette smoke and vehicle exhaust, and in small amounts in foods such as spinach and fruit pits, as well as some prescription medications.

Potassium cyanide is also used for research purposes, said Dr. John Redwanski, director of drug information at the College of Pharmacy at the University of New England.

“It’s not a drug that you could buy legally or that you could get a prescription for,” Redwanski said. “It doesn’t fall under pharmaceutical regulations.”

Potassium cyanide is highly regulated in the United States, but largely unregulated in developing nations, Paloucek said.

U.S. agencies that regulate and oversee the production, use, disposal or cleanup of potassium cyanide include the Department of Environmental Protection, Occupational Safety and Health Administration, Department of Transportation and the Agency for Toxic Substances & Disease.

A person could likely obtain the chemical illegally from a commercial or industrial producer, Paloucek said.

Paloucek noted an increase in the use of cyanide in suicides and high-profile homicides, including the ongoing trial of University of Pittsburgh medical researcher Dr. Robert Ferrante, 66, who’s charged with poisoning his neurologist wife last year with cyanide.

Redwanski said the Maine-related case “is very concerning, especially nowadays with the ability to purchase these substances online and learn how to use them online.”

DEFENDANT HAS TROUBLED HISTORY

The U.S. Attorney’s Office is seeking to have Kilmartin detained while the case is pending. The magistrate judge scheduled a hearing for Nov. 14 at 1:30 p.m. and ordered Kilmartin held at the Somerset County Jail until then. Silverstein plans to meet with him next week to get a better handle on the government’s allegations against him.

“I think the government may have some evidence of when these guys had communicated,” Silverstein said after the court hearing. “It was suggested it was some complicated international mail order transaction.”

Silverstein said the government is alleging that Kilmartin committed the offenses while in either state custody or care after an assault case seven years ago. In that case, Kilmartin was 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.

The man told police that he had been in bed when he heard somebody kicking in his door. When officers arrived at the scene, they found rifles, including one with a scope, in Kilmartin’s pickup truck outside. When questioned about the assault by officers, Kilmartin said he remembered having an argument in the laundry room, but the next thing he recalled was police waking him up.

The victim was treated at the hospital for a broken jaw, broken eye orbital and cheek bone, bleeding on the brain and a possible fractured rib, according to police reports at the time.

Kilmartin was charged with elevated aggravated assault and burglary.

While free on bail, Kilmartin was charged with violating conditions of release after police found beer cans and plastic baggies with cocaine residue in his possession, according to court papers.

At the time, Kilmartin said he was having a difficult time because he was going through a divorce. After a forensic psychological exam, Kilmartin was found not guilty by reason of insanity and committed to Riverview Psychiatric Hospital on Sept. 10, 2008. It’s not clear when or if he was released.

Staff Writers Kelley Bouchard and David Hench contributed to this report.