More searches are planned in a bid to locate missing man John Concannon, 71, has been missing since Tuesday November 10.
The last confirmed sighting is on CCTV footage on a bus at Lenamore Road near the Derry/Donegal border.
Chief Inspector Alan Hutton, Operations Manager for Derry City & Strabane District, said extensive searches have already taken place.
"Both the police service and our Gardai counterparts have been working closely and have carried out searches on land and in local waterways. This has been a joint effort, and we are also are very appreciative of the contributions of a civilian community search team that has assisted.
"The information we have at this stage suggests a search area on and close to the border at Lenamore Road and across into Dundreen and Drumadooey. We will be renewing the searches and will be using specialist search facilities.
"In the meantime, our thoughts are with the Concannon family. They are going through a lot of uncertainty at a time of year when families like to be together.
"I want to thank everyone who has given information so far. I would encourage anyone who thinks they may have seen Mr Concannon, on either side of the border, to get in touch with their nearest police or Garda station.
Edward Concannon realised something was wrong when his father John (71) didn't arrive at his home in the Galliagh area of Londonderry last month.
Although John was extremely fit physically from years of work as a docker and a labourer, he was diagnosed with dementia, added to existing health problems.
Edward has appealed for people living on both sides of the border to be on the lookout for his father.
He said: "We are frantic with worry, our mother died when I was eight and it is just the two of us boys and my father. We are desperate to find him and are so grateful for all the help we have had so far."