The family of a soldier who was found hanged after claiming she had been raped and bullied have said they fear a possible cover-up by the Ministry of Defence (MoD) after new documents emerged on the day the inquest into her death was due to conclude.

Khristina Swain described the family's dismay after the coroner investigating the death of her sister, Anne-Marie Ellement, a corporal in the Royal Military Police, delayed his ruling for a week when MoD lawyers said they had discovered discs containing 1,400 unseen documents. The discs included references to a diary and mobile phones the family say were never passed to them.

"Why haven't we been told about this stuff?" Swain said. "Why has it been held? We just want the truth, that's all. In my eyes, I feel there could be a cover-up."

Lawyers for the MoD insisted the files - only 29 of which are seen as relevant to the inquest - were disclosed as soon as they were located.

Following three weeks of evidence at the inquest in Salisbury, Wiltshire, the coroner, Nicholas Rheinberg, had been scheduled to deliver his decision on the death of Ellement, who was found hanging in her barracks in October 2011, shortly after her 30th birthday.

But the decision was postponed after lawyers for the military disclosed the discovery of two discs containing the files over the weekend.

Nicholas Moss, the barrister for the Ministry of Defence, said the great majority of the files were connected to Ellement's military police casework. But there were also files such as Ellement's covering letter for her application to be transferred to the Royal Army Veterinary Corps, and a note about a planned interview, scheduled to take place the day after she died.

Also among the newly found files was an inventory of items found in Ellement's room at Bulford barracks near Salisbury after her death, produced by the army's Joint Casualty and Compassionate Centre. The court heard this listed three mobile phones and a pink-covered 2010-11 diary, the latter referenced with a note that one page contained an entry that could be "sensitive and upsetting" for the family.

Kirsten Heaven, the barrister acting for Ellement's sisters, said they had never previously seen the inventory or been passed the phones or the diary. She said: "The family are devastated and upset that these documents have come so late in the day."

Heaven cited guidelines on the disclosure of evidence at inquests, saying the family believed the MoD had clearly breached these by not producing the documents until the very end of the inquest, by which point all witnesses had been cross-examined.

Speaking after the hearing, Swain said: "I'm so angry and so upset after waiting all this time, especially a year, to get where we are, to find out we haven't had all the information and documents we actually need – I'm just absolutely devastated.

"The good thing is, by waiting until next Monday, hopefully we'll get all the answers we need and hopefully we'll finally get the truth."

The campaign group Liberty, which has been assisting the family, said it was "impossible to overstate" the impact of the revelation. Emma Norton from Liberty said: "Despite their understandable anger and grief over the circumstances of her death, they approached this inquest in good faith, trusting the MoD to finally do the right thing. That faith now seems misplaced, and the need for independent oversight of the army's justice system becomes more glaring with each fresh failure."

Moss said the omission was accidental and the MoD had informed the coroner and family as soon as it had made the discovery, late on Friday. He said: "There is no suggestion at all of any wrongdoing." He added: "The MoD has gone to exceptional lengths to seek to provide as full disclosure as possible."

Moss said it was possible the diary and phones had been passed to Ellement's father, Kenneth. Ellement's sisters are not in touch with him, and police will seek to find out whether he has them, the inquest was told.

Rheinberg adjourned the inquest for a week but warned he was not willing to wait "forever and a day" for evidence that might be both impossible to track down and of possibly limited relevance.

The inquest has heard that Ellement felt downcast after military investigators decided to not prosecute two soldiers she said had raped her in November 2009 while she was posted in Germany. Her other sister, Sharon Hardy, said Ellement had been left "absolutely devastated" by the decision.

A former colleague, Corporal Charlotte Berrill, told the inquest that Ellement felt bullied by other soldiers in Germany who accused her of "crying rape" and she had sought to return to the UK.

The inquest is the second into her death. The family secured a new hearing to look into allegations that she was bullied and overworked before her death.

The inquest has heard that Ellement was placed on a suicide-risk register in Germany but this was not passed to officers when she was transferred to Bulford.

Hardy told the court that when she heard her sister was dead her immediate thought was that this had happened because of "the army, the rape, the bullying and the overwork".

The inquest continues.