Stockholm  - Swedish police Thursday continued the hunt for criminals that used a stolen helicopter and explosives to raid a cash depot early the day before, but did not report any arrests.

The robbers secured a headstart by placing what was believed to be a bomb outside the hangar entrance at the Stockholm police helicopter base, preventing the police helicopters from taking up pursuit.

Police were criticized for poor security at the base that relied mainly on camera surveillance and movement detectors.

National Police Commissioner Bengt Svensson said the base was a temporary solution and police were working to find a permanent location, perhaps at a military base.

Police spokesman Ulf Goranzon said several hundred tips had been received and were being analysed as part of the probe. In addition, police forces in other countries had been contacted via Interpol and Europol.

The daring raid suggested that some of the suspects likely had military training, but police investigators declined to speculate.

The value of the money stolen has not been disclosed, but security services company G4S, which operated the cash depot, has posted a reward.

Police hoped to secure DNA and other forensic evidence from the helicopter, which was found abandoned in a field north of Stockholm.

Possible clues could also be found at the cash depot and the aviation school in Norrtalje, 70 kilometres north of Stockholm where the helicopter was stolen.

Aviation experts said the pilot who flew the helicopter was likely very skilled, citing the pre-dawn flight to the depot building and the location of landing site close to high trees.  dpa