Allan Michael Grimson was born in 1958 in the United Kingdom and served as a career Royal Navy officer for more than two decades. He enlisted in the Royal Navy in 1978 and rose to the rank of Petty Officer (PO), achieving considerable authority within naval fire-fighting training programs. By the late 1990s, Grimson was stationed at HM Naval Base Portsmouth and was assigned to HMS Excellent, a shore establishment where he instructed fire-fighting techniques for cadets and trainees. His duties provided him unfettered access to young recruits, which, as later admitted, he used to target and exploit those he found physically attractive.

Grimson had also served aboard the aircraft carrier HMS Illustrious, including during its docking in Gibraltar in 1986—a connection that would become significant years later in a separate missing persons case. Known to be strict and commanding in demeanor, his authoritative role masked deep psychological disturbances. When questioned by police years later, Grimson admitted he had actively "scouted" young male sailors—those he considered the best-looking—for dominance and violence, rather than any consensual relationship. He admitted that he found pleasure not in sex, but in control, pain, and power.

Psychological assessments later identified Grimson as possessing traits consistent with extreme psychopathy. One clinical psychologist who reviewed Grimson’s case described him as "the worst psychopath I have ever come across in 250 murderers." Despite this, Grimson maintained a respectable naval career for years, hiding his compulsions behind his disciplined military facade.

Grimson’s descent into violence was first exposed in December 1999 when he confessed to murdering two young men—both Royal Navy associates—on the exact same date, one year apart: 12 December 1997 and 12 December 1998. His first known victim was 18-year-old Nicholas Wright, a Royal Navy sailor he had met during a firefighting course in Portsmouth in 1997. Grimson offered to drive Wright home to Leicester multiple times, eventually gaining the young man’s trust. While Wright’s family was cautious of Grimson’s motives, the teenager continued to associate with him.

On 12 December 1997, while Wright was on shore leave from HMS Edinburgh, he was seen drinking with Grimson in a Portsmouth nightclub. They returned to Grimson’s flat, where Grimson made sexual advances. When Wright rejected him, Grimson launched into a brutal attack—first punching him, then bludgeoning him with a baseball bat, and finally slitting his throat with a carving knife. He later disposed of Wright’s body along the A272 road in Hampshire. Initially questioned by military police, Grimson denied involvement and evaded arrest.

A year later, on 12 December 1998, Grimson repeated his killing. His second confirmed victim was 20-year-old Sion Jenkins, a barman who had briefly served in the Navy. Jenkins met Grimson at Joanna’s nightclub in Portsmouth. Intoxicated, Jenkins accompanied Grimson back to his flat. There, Grimson forcibly engaged him in sexual activity under threat and violence. When Jenkins tried to leave the next morning, Grimson tied him up in the bath and beat him to death with a baseball bat—again chasing the same "thrill" he experienced the year prior. Jenkins’ body was dumped off the A32 road near West Tisted in Hampshire.

Grimson was finally arrested in December 1999 and confessed to both murders in disturbing detail. He claimed the thrill of killing Wright had exceeded any sexual satisfaction, calling it "better than an orgasm." In March 2001, Grimson was convicted of both murders. Justice Cresswell called him "a serial killer in nature if not in number" and said he posed an enduring risk to society. Originally sentenced to a minimum of 22 years, the term was later increased to 25 years by the Home Secretary. In 2008, three years were deducted from his sentence to account for time served on remand.

Police continue to suspect that Grimson may have killed others. The precision with which the two known murders occurred—on the same date a year apart—led investigators to suspect a ritualistic or commemorative element. Authorities believe there may be as many as 11 to 20 additional unidentified victims. A high-profile unsolved case linked to Grimson is the disappearance of Simon Parkes, a 20-year-old naval rating who vanished in Gibraltar on 12 December 1986. Both Parkes and Grimson served aboard HMS Illustrious, and witnesses placed them together on the night Parkes vanished. Although Grimson denies involvement, police flew to Gibraltar, searched sewers, and brought in cadaver dogs—but no trace of Parkes has ever been found. The BBC covered the case in its 2005 special The Search for Simon Parkes, sparking renewed public interest and criticism from Grimson, who claimed unfair media treatment.

Another murder possibly linked to Grimson is that of 29-year-old Kayo Matsuzawa, a Japanese woman found dead in a stairwell cupboard in Auckland, New Zealand, in 1998. At the time, Grimson was in the country on a Royal Navy exchange program. Although he was seen in the building where Matsuzawa's body was found, no formal charges have been filed, and the victim does not match Grimson’s usual profile of young male sailors.

In 2019, it was revealed that Grimson had been transferred to an open prison. In 2022, a parole board confirmed that he would have an oral hearing to assess his suitability for release. This prompted fierce opposition from victims' families, particularly Margaret Parkes, mother of missing sailor Simon Parkes. She expressed strong concerns about Grimson ever being released, stating, "He’s a dangerous person... How can someone with that mentality change?"