A risk assessment was being carried out about the data that was missing, alongside the internal inquiry into what had happened, she said.
And no more information was being passed to the firm while the investigation continued and the government was "reviewing the terms of that contract and other contracts" with PA Consulting.
The memory stick contained un-encrypted details about 10,000 prolific offenders as well as names, dates of births and some release date of all 84,000 prisoners in England and Wales - and 33,000 records from the police national computer.
The Conservatives say the Home Office appears "incapable" of keeping data secure and warn criminals may seek compensation.
PA Consulting has searched its premises and looked at CCTV recordings in an attempt to recover the missing memory stick - a commonly used portable storage device for computer files.
The company was handed data as part of a research project on tracking offenders through the criminal justice system.
WHAT IS A MEMORY STICK?
Also called USB sticks or flash drives
Have almost completely replaced computer disks
Used to carry and transfer data easily
Data stored on tiny chip instead of magnetic disk as with hard drives
Holds up to 32 gigabytes of data, equivalent to 32,000 large books
A £1 stick could hold more than 10 pages of data on each prisoner
Government departments were ordered to tighten up their security procedures after the loss of two discs containing personal details of every child benefit claimant in November.
But shadow home secretary Dominic Grieve said the latest data loss showed the Home Office was "entirely incapable of keeping it secure".
And he said there was a serious risk that if criminals' details were found by a third party, they could sue the government for compensation.
"It is ultimately the Home Office's responsibility to maintain the security of this material," he added.
PA Consulting is one of the companies that has been involved in developing the government's controversial ID card scheme - opposed by the Conservatives and Liberal Democrats.
Lib Dem leader Nick Clegg said: "The government will no doubt seek to blame private contractors, but the rash of data losses over the last two years confirm that there is something much more worrying at stake: this government cannot keep any information safe."
FROM THE TODAY PROGRAMME
David Smith, Deputy Commissioner in the Information Commissioner's Office, said the latest loss showed that personal information could be a "toxic liability" if not handled properly.
"It is deeply worrying that after a number of major data losses and the publication of two government reports on high profile breaches of the Data Protection Act, more personal information has been reported lost," he said.
Labour MP and chairman of the home affairs select committee Keith Vaz told BBC Radio 4's Today programme he hoped the government had put adequate safeguards in place.
"If you hand out memory sticks almost like confetti to companies and ask them to do research for you, then you have to be absolutely certain... that the company concerned has put in practice procedures which will be just as robust as the procedures that I hope the government has followed," he said.
A spokesman for PA Consulting refused to comment.
Earlier this month the BBC apologised after a memory stick containing the personal details of hundreds of children who had applied to take part in a TV show was stolen from a vehicle.
On Tuesday, a BBC analysis found sensitive data potentially affecting more than four million people had been lost by government departments in the year to April.
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