A fifth person has been arrested in the United States during an ongoing FBI investigation into an extreme Jewish sect and a family that fled it.

The FBI said the detained man had been planning to abduct two children of a former member of the Lev Tahor sect, which is now based in Guatemala.

The mother had left Lev Tahor, which her father founded, and last year went to live in New York with her 12-year old son Chaim and her 14-year old daughter Yante. But in December the children were kidnapped by sect members.

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They and their captors were found by police in Mexico three weeks later and returned to their mother in New York. Four people were arrested.

But the sect’s leader has said he will “fight” the mother “until the last drop of blood,” adding that supporters in New York will continue to try to kidnap the children.

The 230-strong sect had initially been based in Canada until 2014, before raids by social workers uncovered child abuse on a massive scale, with under-age marriage, sexual abuse, the forced ingestion of drugs and squalid living conditions.

Children were found to be living in filthy conditions, with social workers describing how up to five children were forced to sleep on the same urine-soaked mattress.

A judge deemed dozens of the sect’s Charedi children to be at risk of serious psychological and physical abuse and ordered the group members not to leave the country, but they turned up in Guatemala days later.

The criminal complaint says Lev Tahor considers the 14-year-old girl to be the wife of Jacob Rosner, one of the men charged in the December kidnapping.