Unmarked computer discs have been sent to three city centre property firms.

Hackett Property, on Frederick Street, was the first company to receive the discs, which appear to contain sections of Sunderland's electoral register from the 1950s and 60s.

After an appeal in the Echo, two other firms came forward.

Halifax Property, on Anthenaeum Street, was also hand-delivered a disc, but did not open it on the advice of Halifax head office.

And now Manor Property, also on Frederick Street, have received a copy of a similar disc.

Janet Stafford from Manor Property contacted the Echo to say she has CCTV cameras which could have captured the person who posted the bizarre package, which arrived last month.

She said: "The cameras cover the front door, which is where the disc was posted. We don't know who sent it, or what the information on it means. It was a complete mystery, until we saw the piece in the Echo."

Staff from all three companies said they had no idea why the discs have been sent to them.

A Sunderland City Council spokesman said information on the electoral register was freely available to members of the public.

Police have advised anyone who receives the disc to contact Sunderland Area Command on tel. 08456 043043.

IT experts would warn against opening unknown computer discs. Computer viruses can be transmitted through floppy discs or CDs.

A virus is a computer program which can damage or even delete files. They can also spread through office computer systems, taking up memory and causing systems to crash.

Can you solve the riddle of the mystery discs? Contact Jane O'Neill at the Echo, tel. 565 8557.