Mystery still surrounds a break-in at the parliamentary offices of the Labour leadership as party sources indicated that it was "deeply concerned" about the security breach.
Police were called out on Friday evening shortly before 7pm when an aide found a door to a room which forms part of a suite used by Ed Miliband and his team had been forced.
It was initially linked by some observers to a series of recent thefts of laptops and other equipment from offices across the Westminster estate.
But a senior Labour source said the motivation behind the break-in remained to be established and that it was still unclear if anything had been taken.
"We do not know whether anything was taken. We do not know the motivation for this crime," the source said.
"But we are deeply concerned that this can take place within Palace of Westminster."
Mr Miliband and senior aides had spent the day in the Midlands at a party conference.
It was reported that the targeted room was a "policy office" not far from Mr Miliband's own in the Northern Shaw buildings - the home of New Scotland Yard up until 1967.
A Metropolitan Police spokesman said: "Police were contacted at 6.55pm regarding an alleged forced entry to an office in the Palace of Westminster. Inquiries continue."
The Commons heard recently that thieves steal two computers a month from Parliament, one of the most heavily-guarded buildings in the country. Ten vanished in May last year alone.
One belonged to Home Affairs Select Committee chairman Keith Vaz who also lost an iPad, causing him to call for "a more robust approach to security inside the building".