Campaigning families of Omagh bomb victims are preparing for a courtroom showdown with the Government after it rejected their demand for a public inquiry into the Real IRA attack.
Relatives challenged the UK authorities to "stop hiding from the truth" over alleged intelligence and investigative failings they claim allowed the bombers to perpetrate the 1998 atrocity, and get away with it.
Ms Villiers said she did not believe there were sufficient grounds to justify a state-commissioned independent inquiry.
The dissident republican attack which killed 29 people, including a woman pregnant with twins, and injured hundreds more was one of the worst atrocities of the Northern Ireland troubles and inflicted the greatest loss of life in a single terrorist incident.
With the bomb having been allegedly transported to the Co Tyrone town from the Republic of Ireland, bereaved families have called for a cross-border inquiry involving both the London and Dublin governments.
Irish Taoiseach Enda Kenny tonight said his Government was still considering a report the families had compiled on the alleged state failures north and south.
Stanley McCombe, whose wife Ann, 45, was killed, said the anger he felt at the British Government's decision would drive him onward as the families proceeded with legal action.
"If they want to try and hide the truth about Omagh, they can," he said.
"But we'll flush them out at the end of the day. There are no hiding places. It's a democratic country and people have to know the truth."
Mr McCombe vowed never to give up on the campaign. "I will not leave it until the day I die," he said.
Michael Gallagher, whose 21-year-old son Aiden was among the victims of the August 1998 blast, added: "We'll do our talking in court."
But not all the Omagh bomb families want an inquiry, with some claiming the exercise would re-traumatise the bereaved.
Kevin Skelton, whose wife Philomena, 39, was killed, said: "I am making my position quite clear and I would have other families behind me in that, who are not interested in a public inquiry because they don't think it's going to achieve anything."
Omagh was bombed just months after politicians in Northern Ireland signed the historic Good Friday peace accord that led to power-sharing at Stormont.
While no-one has been criminally convicted of the crime, four republicans were found liable for the atrocity in a landmark civil case taken by some of the bereaved relatives and ordered to pay £1.6 million compensation.
Last month, families pressing for the inquiry, many of whom belong to the Omagh Support and Self Help Group, outlined details of an independent report they commissioned into alleged intelligence mistakes in the lead-up to the atrocity and gaffes in the subsequent criminal investigations.
They had handed the document to the authorities in London and Dublin a year previously and complained vociferously at the length of time the respective governments had taken to respond.
Ms Villiers said ruling out an inquiry was not an easy decision to make and all views were carefully considered.
She said a current investigation into elements of the incident by the Northern Ireland Police Ombudsman was the best way to proceed.
"I do not believe that there are sufficient grounds to justify a further review or inquiry above and beyond those that have already taken place or are ongoing," she said.
Ms Villiers added: "The fact remains that the Real IRA carried out the bombing in Omagh on August 15 1998, murdering 29 people and injuring many more.
"Responsibility is theirs alone. I sincerely hope that the ongoing police investigation will bring to justice those responsible for this brutal crime."
Ms Villiers acknowledged that families had contrasting views on an inquiry, with some believing it would cause them considerable trauma.
She said all the views were weighed against other factors, including the series of previous inquiries into the Omagh bomb and the current investigation by ombudsman.
Both Mr Gallagher and Mr McCombe questioned the rationale of the Government's claims that an inquiry would cause trauma to some of the bereaved.
"Of course we recognise that people move forward at different levels but does that mean to say that because there are some of us who want justice and truth that we should be denied that because others don't?" said Mr Gallagher.
Mr McCombe added: "The way I look at it is: why should anyone else deny me the truth about why my wife was murdered?"
Expressing concern at the length and cost of the public inquiry into the Bloody Sunday killings by British Army soldiers in Londonderry in 1972, the Government had previously made clear its resistance to holding further such probes into Troubles incidents in Northern Ireland.
Mr McCombe said the experiences of the Saville Inquiry in Derry, which cost about £200 million and ran for more than 10 years, did not have to be repeated.
"If everyone co-operates, then there is no problem," he said.
"The only reason people won't co-operate is because they don't want the truth to be known."
He added: "Why should anyone in Government, why should they have the right to keep the truth from me and my family?"
Mr Gallagher said a public inquiry was needed because previous probes had been unable to compel people to give evidence.
Former Northern Ireland police ombudsman Dame Nuala O'Loan, who while in office carried out her own investigation into the bombing, and former Metropolitan Police assistant commissioner and counter terrorism chief Bob Quick have publicly backed the call for an inquiry.
Amnesty International has also added its voice to demands for a full independent probe.
But a number of the families are not in favour.
Mr Skelton said his children were of the view that their mother should be allowed to rest in peace.
"But I am not standing in the way of anybody, of the other families going for what they believe in, I have never done that," he said.
" We know the answers. I know there were dirty deeds done round Omagh and the Government, whether there is a public inquiry or not, they are going to bury them, and they have the power to do that."
Mr Kenny said the Irish Government was still examining the claims made in the families' report.
"The Government are in possession of the report from the families and the minister for justice (Alan Shatter) is considering that," he said.
Mr Kenny stressed he was prepared to meet and listen to concerns raised by Troubles' victims, and not just those from Omagh.
The Republic of Ireland's deputy prime minister Tanaiste Eamon Gilmore said: "We are hugely conscious of the continuing suffering of the families following the horrific bomb that was let off in Omagh."
He added: "Both jurisdictions will continue to pursue those who were responsible for the Omagh bomb, to bring them to justice."
UK shadow Northern Ireland secretary Vernon Coaker said he hoped the Police Ombudsman probe could provide the families with further answers.
" I await the outcome of that investigation and hope it can help bring the families closer to the truth and justice they rightly demand and expect," he said.