The senior officer in the Omagh bomb case "does not believe MI5 withheld any intelligence from the PSNI".
Senior officers met with the Omagh bomb victims' families last week
Chief Constable Sir Hugh Orde said the lead detective confirmed this to him.
After a police briefing to Omagh relatives last week, questions were raised about whether MI5 failed to pass on tip-offs from an American agent.
David Rupert said dissident republicans planned to carry out bomb attacks in Londonderry or Omagh, but Sir Hugh said this involved a different unit.
Sir Hugh Orde was addressing the final public session of the current Policing Board.
New members are expected to be appointed to the board on 1 April.
The chief constable refused to confirm if MI5 held back any information months before the 1998 atrocity in which 29 people died.
The SDLP said failure to directly answer the question "will not reassure people".
SDLP board member Alex Attwood said: "The truth of the matter is there may have been intelligence prior to the murders that wasn't shared.
"We will never know if that might or might not have avoided that awful tragedy."
'State of the inquiry'
Speaking at Wednesday's board meeting, Sir Hugh said he would not comment on reports about whether MI5 passed information to police prior to the bombing.
"It's the view of the senior investigating officer (Superintendent Norman Baxter) - who I spoke to only two hours ago - that the security services did not withhold intelligence that was relevant or would have progressed the Omagh inquiry."
Dissident republican suspects investigated in April 1998 were from a different cell than those involved in the Omagh bomb plot, said Sir Hugh.
"There's no evidence to link these two units," he said.
The chief constable confirmed senior officers had met with the Omagh bomb victims' families last week to brief them on the state of the inquiry.
Last year, County Armagh man Sean Hoey was the first person charged with murder in relation to the bombing.