The official police watchdog investigation into the shooting dead of Mark Duggan by a firearms officer has found no evidence of any criminal offence, in the clearest sign yet that it is set to conclude the killing was lawful, the Guardian has learned.

The shooting of Duggan on 4 August 2011 in Tottenham, north London, triggered riots that spread across the capital and to other cities. The second anniversary of his death will be marked by his family on Sunday with a vigil at the spot where he was shot twice by a police marksman.

The family are angry that the official inquiry conducted by the Independent Police Complaints Commission is yet to be finalised and released to them.

In a clear sign that the IPCC believes there is no evidence of criminal wrongdoing uncovered by its investigation, no officer has been interviewed under caution for potential criminal or disciplinary offences, with the investigation virtually completed. On Friday an IPCC spokesperson confirmed: "We have found no evidence to indicate criminality at this stage."

An IPCC commissioner will decide whether to refer the case to prosecutors once the report is finalised, but so far no advice has been sought on charges from the Crown Prosecution Service.

The IPCC says its final report will be finished this month. It is waiting on one report from an expert, but otherwise proactive work on the investigation has ended.The final say on the lawfulness or otherwise of the shooting will be delivered by an inquest jury which will start hearing the case next month.

New tests and other work have been ordered by inquest officials into the shooting, above and beyond those carried out by the IPCC. A new expert report originally commissioned by the family is understood to challenge aspects of the police account of the shooting.

The team behind the Duggan inquest is the same that overturned an earlier IPCC exoneration of the Met in another shooting death. The IPCC findings in the 2005 death of Azelle Rodney were overturned after an inquiry produced new evidence and expert testimony that supported a finding that he had been unlawfully killed after a police marksman opened fire.

None of the 11 firearms officers at the scene of the Duggan shooting who were asked to attend interview have answered oral questions from the IPCC, instead supplying written answers. All initially refused to attend interview, and only the officer who shot him later attended. The officer, known as V53, declined to answer questions orally, instead submitting written answers two days later.

V53 has said his substantive account of the shooting was compiled three days later, with he and his colleagues spending more than eight hours sitting in a room together writing their statements. He says he has "no doubt" Duggan had a gun and was preparing to open fire.

Neither Duggan's DNA nor fingerprints have yet been recovered from the weapon or the sock it was contained in. The weapon was found 10ft to 14ft from where he fell, over a low fence, after he was shot twice.

Marcia Willis-Stewart, solicitor for the Duggan family, said: "The family would say the IPCC have been tried, tested and found very wanting. We do not have a draft report or final report and don't know when we are getting either from the IPCC."

Duggan had been trailed by officers on the day he was shot and was riding in a taxi which was forced to a halt by armed officers. They believed he had collected a gun. A man has been convicted of supplying the weapon to Duggan.

The IPCC had to correct the initial information it released, which came from the Met but which it adopted, saying Duggan had fired and that a bullet had lodged in a radio worn by a police officer. The IPCC later admitted the bullet was in fact most likely a ricochet from one fired by a police officer.

The IPCC was criticised by Tottenham MP David Lammy, who said: "It is a scandal. There have been rapists terrorists and murderers who have gone to trial in the two years we have had to wait for Duggan. They have not kept the community informed. They have a public guardianship role and they have failed."

"It makes all our jobs harder that the IPCC appears to be so ineffective."

There will be a public meeting this eveningand a vigil tomorrow organised by the Duggan family at the spot where he was killed, at the exact time he was shot two years ago.

Police have drawn up contingency plans but both their intelligence and community sources do not suggest tensions spilling over into serious disorder.

The Metropolitan said: "An appropriate policing plan is in place for a number of events taking place across the capital this weekend. Extra police officers are on duty and they will be made available to Haringey borough, should they be required."

The IPCC is also investigating police over the weapon Duggan allegedly had after it emerged it may have been used a week earlier in an assault by another person.