A time-critical investigation into the death of a British man whose body was found in a ransacked property on the Caribbean island of Grenada has stalled because of diplomatic delays between the two countries.

Charity workers supporting the family of Andrew Frederick are demanding immediate government intervention to clear the path for a full investigation to begin almost 10 months after his death, which has been ruled a homicide by a pathologist.

The Murdered Abroad charity has called on the foreign affairs select committee to step in after Grenadian police found there was “no basis … that any homicide has occurred” and closed the case.

This was despite a report from an autopsy, done in the week he died, saying Frederick, 47, had suffered “blunt force trauma”, was tortured, including by removal of his toenail, and ruling the death a homicide.

Frederick’s body was found on 4 January. Since then, the family say they have been forced to focus on their fight for justice instead of coming to terms with their loss.

Murdered Abroad’s founder, Eve Henderson, said:“We understand there are challenges but [almost 10 months] on, this family is still being met with obfuscation. If a family this determined cannot get the FCDO [Foreign, Commonwealth and Development Office] to put their diplomatic powers to good use, what hope is there for the 60 to 80 families bereaved by murder abroad each year?

“A minister must look urgently at how consular services treat these traumatised families or the foreign affairs select committee must step in,” she added.

A former Scotland Yard homicide commander who is supporting the family described Grenada’s actions as “incredible”.

“In the autopsy report … the pathologist, who is a qualified medical examiner, categorically states that he believes this is a case of homicide and that Andrew was tortured before he was killed,” David Johnston said. “And yet the police just choose to ignore that because it didn’t fit with their model. I mean, it’s incredible.”

After a sustained campaign by the family, British authorities became involved and Frederick’s body was repatriated to the UK in August, where a second autopsy was carried out, during which it was discovered his organs were missing.

The family described this discovery as “shocking and devastating”.

In a statement, they said: “We were already heartbroken that Andrew’s life was cruelly taken, but nothing could have prepared us for what we would learn afterwards. For six weeks after his death, Andrew’s body was held under the control of the Grenadian authorities. When he was later examined in the UK, parts of his neck and all of his organs, including his brain, heart, and lungs, were missing.

“That discovery has been shocking and absolutely devastating for our family; we are still struggling to process it. It has added pain upon pain and left us with deep hurt with so many unanswered questions. Despite our multiple inquiries, no one has told us where they are, what happened to them, or why we were never told. No family should ever have to ask these questions.”

Last month a UK coroner asked the Metropolitan police to conduct an investigation. Weeks later a communication from UK authorities, seen by the Guardian, said a British high commissioner had raised Frederick’s case with officials at the highest levels of government in Grenada, and indicated that authorities there had agreed to cooperate with any forthcoming formal request for the UK to assist with the investigation.

That formal request was sent weeks ago but the family say they are still waiting for Grenada “to honour their verbal commitment to the British high commissioner and approve the request to cooperate”.

“Every day for almost 10 months we have had to live without Andrew and without answers. A formal request from the UK, asking Grenada to cooperate with its investigation into Andrew’s death, still sits unsigned. It is the best chance we have of finding the truth,” the family statement said.

Johnston said the initial handling of Frederick’s case and the further delays could have a detrimental effect on the investigation.

“In a homicide investigation we have a phrase that we refer to as the golden hours, which are the first real 24 hours from the notification, or the discovery, of a homicide [which] are the most important and crucial … because evidence diminishes and can be lost if a crime scene is not properly noted and properly managed.

“In the case of the death of [Frederick] that’s exactly what happened … the police, in my view, failed to properly identify and preserve the crime scene and to treat the crime scene in the manner it should have been treated,” he said.

Johnston added that cooperation between British and Grenadian police could help to “bring some stability to that investigation”.

An FCDO spokesperson said: “We are supporting the family of a British man who died in Grenada, and are in contact with the local authorities.”

The Guardian has approached the Royal Grenada Police Force for comment but has not had a reply.

The family said: “With each passing day, our family’s heartbreak deepens. We need closure, and Grenada’s prime minister, who is also the minister of national security, can help make this possible. We appeal to him, with humanity and compassion, to sign the request without further delay.”

Additional reporting by Calistra Farrier