Investigators can open the most sensitive files in a controversial case that centres on a Swiss family suspected of dealing in nuclear secrets and espionage.

Switzerland's justice ministry has confirmed reports over the weekend that it will allow lawyers and investigators access to documents in the so-called Tinner affair. The papers contain nuclear bomb plans and may provide links between Swiss engineers and the United States Central Intelligence Agency (CIA).

Urs, Marco and Friedrich Tinner have been the subject of allegations that they contributed to a nuclear smuggling ring run by Abdul Qadeer Khan, the father of Pakistan's nuclear programme, that supplied Libya with weapons components. The Tinners have maintained their innocence.

At the centre of the investigation is a dossier that contains roughly 100 pages of documents so sensitive that the government wants them destroyed. The original files were secretly shredded in November 2007 but copies surfaced last December.

In July, investigators and Bern cantonal police raided federal offices which housed the copies in a move that exposed tension between Switzerland's executive and judiciary branches. Lawyers, parliamentarians and the government were then locked in a showdown over what should happen to the documents.

Last week, the federal criminal court opened the way for magistrates to look at the files, provided federal ministers gave their authorisation.

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