An American who had abducted his children and brought them to Malta, is to receive a five-year prison sentence after pleading guilty to a Houston court of attempting to sell a bomb to undercover FBI agents posing as members of terrorist group al Qaeda.

In return for building the bomb, 70-year-old Ronald Grecula, an engineer by profession, wanted help to have his estranged wife Monique killed or put in jail to regain custody of his two children, Emilie and Berenger.

Grecula had brought them over to Malta in 2002. A court case was instituted after the mother was given an anonymous tip-off and she filed an application for a prohibitory injunction against him to stop him leaving the islands with the children.

After a heated and lengthy legal battle, the Maltese courts awarded custody of the children to the mother and eventually charged Grecula with overstaying his visa and other indictments.

He returned to the US, deeply resentful of the way the US authorities had dealt with the case. Grecula’s ability to build a superbomb using a dubious theory of fusion remains in doubt, but not his intention of selling his technology to al Qaeda.

He pleaded guilty in a Houston federal court on Thursday to charges of offering to provide the terrorist organisation with an explosive device that he claimed was nearly as powerful as a nuclear bomb.

As it turned out, the crackpot engineer was dealing with undercover FBI agents and Houston police officers posing as al Qaeda members.

Grecula also admitted offering to teach a ground school for al Qaeda pilots and telling an undercover agent that “he had a dislike for America”.

He was also indicted in August this year on a drug charge, accused of trying to buy heroin to plant on his estranged wife, Monique.

Grecula had discussed hiring someone to kill his wife “in a mysterious way” to avoid it looking like murder and regain custody of their two children, according to an affidavit.

In return for his guilty plea to one count of providing material assistance to a terrorist organisation, Grecula will receive a five-year prison sentence. The maximum penalty is 15 years and a $250,000 fine. He is to be sentenced on 15 December.

The bomb that Grecula had claimed he could make would have employed fusion technology (which does not even exist) to build a super bomb that could level a city.

However, investigators did find bomb-making materials in his home.