OAKLAND — On December 3, 41-year-old Sanford Faison allegedly logged onto his computer and posted a question on an anonymous Dark Web forum. His tone was matter-of-fact, as if he was asking for help on his car engine, or the ideal temperature to cook a steak.

But Faison had a much different purpose in mind: According to the FBI, he was trying to find the best way to murder his wife with an unnamed, extremely lethal chemical weapon he was attempting to buy online.

“I aim to ensure my wife’s death within the 18 months, ideally long after our divorce is finalized (about six to eight months from now),” Faison wrote, according to a recently unsealed criminal complaint filed in federal court. “This is the only way I can begin a new life with full custody of my child. We currently live together, and I expect to have (easy) access to her environment and food for another two months at least.”

Faison was arrested and charged Jan. 9 with attempting to acquire a chemical weapon. The charging documents, unsealed Thursday, remain secretive: Authorities did not release the name of the chemical, describing it as “a colorless, volatile, flammable and highly toxic liquid” that is “easily absorbed through the skin and may produce life-threatening systemic effects with only a single drop.”

Similarly, the website Faison visited to acquire the chemical, as well as the anonymous Dark Web forum, were both kept under seal.

“Because of its high toxicity, the [toxic chemical] has very few legitimate applications and is primarily used in chemical research. It can be used to fatally poison humans,” an FBI agent wrote in the complaint.

Faison had reportedly theorized several ways he would make his wife ingest the chemical: sneaking it into her coffee, using an eyedropper to dab some on her while she slept, or sticking some on a doorknob or keyboard.

Because of the nature of the chemical, the investigation was handled by a member of the FBI Bay Area’s anti-terrorism squad. Federal authorities say Faison arranged to buy a 10 milliliter vial of the chemical through the Dark Web, a section of the internet where people can anonymously buy and sell illicit goods.

On the forum postings, Faison made his motives clear, saying he wanted to kill his wife of eight years — with whom he had two children — but noted that he would be a primary suspect “if she so much as slipped on a banana peel,” according to the complaint.

“I feel that [chemical #1] poisoning would reduce my culpability by delaying a final/accurate diagnosis and undermine any investigation,” he allegedly wrote.

Faison allegedly found a prospective seller and arranged for the chemical to be sent in a hidden package to his tenant in San Leandro. The purported seller, unknown to Faison, was working for the FBI.

Federal authorities sent a package to Faison, but it contained surveillance equipment. When Faison opened the package, he was arrested. Later, he admitted to the plot, calling his actions “premeditated” and saying he originally wanted to hire a hitman but determined that would cost as much as a divorce.

Faison worked at PayPal and had no legitimate reason to acquire the chemical, FBI agents wrote in the complaint. He remains in federal custody, awaiting trial.