Martin McGartland was shot as he sat in his car in Duchess Street, Whitley Bay, on 17 June 1999.
Five people have been arrested but no one has ever been charged.
Northumbria Police said its main line of inquiry continued to be that he may have been shot by someone with Irish Republican sympathies.
Mr McGartland survived despite being shot several times at close range and was given a new identity.
Det Ch Supt Chris Thomson said: "Although this was an unsuccessful attack, it was a cold-blooded, calculated assassination attempt."
He added: "Mr McGartland's history as an agent for the then Royal Ulster Constabulary and his supplying of information about the Provisional IRA is a matter of public knowledge, documented in his own books on the subject.
"Our main line of inquiry, therefore, continues to be that Mr McGartland may have been shot by a person or people with Irish Republican sympathies for reasons closely linked to his former life in Northern Ireland."
Northumbria Police said it was continuing to use DNA recovered from the scene to eliminate people from the investigation.
Other lines of inquiry concern a van abandoned at the scene which had been bought only two days before the attack and a message left on a telephone answering machine a few days earlier asking about a van for sale.
Police said two semi-automatic pistols and some ammunition were found in the Gateshead area within months of the attack and one was forensically linked to the shooting.
Police have renewed their appeal for information 11 years after the attempted murder of a former IRA informer on Tyneside.