A federal judge on Thursday blocked Elon Musk’s so-called “department of government efficiency” (Doge) from accessing social security records as part of its hunt under Donald Trump for fraud and waste, calling the effort a “fishing expedition”.

Judge Ellen Hollander granted a temporary restraining order that prevents Social Security Administration (SSA) workers from allowing Doge to have access to records that contain personally identifiable information.

Musk, the world’s richest man and a huge political backer of Trump, has been tasked by the US president with slashing costs and employees at the federal government: a mission that has caused chaos and disruption across the US amid mass firings and huge numbers of government projects and contracts being canceled.

The Trump administration says Doge has a 10-person team of federal employees at the SSA, seven of whom have been granted read-only access to agency systems or personally identifiable information.

The lawsuit challenging Doge’s access to sensitive records was brought in February by the American Federation of State, County and Municipal Employees, the Alliance for Retired Americans and the American Federation of Teachers.

Attorneys for the government argued the Doge access did not deviate significantly from normal practices inside the agency, where employees are routinely allowed to search its databases. But attorneys for the plaintiffs called the access unprecedented.

In her ruling Hollander also instructed Doge to “disgorge and delete” any non-anonymized data it has obtained from the SSA since Trump took office, and said the agency cannot install or access any software in social security systems.

Social security payments are a lifeline for millions of elderly Americans across the country and any effort to cut back the system is widely seen as a political minefield. However, Musk has claimed the system – without providing much convincing evidence – is rife with fraud.