He was thought to have hired the house used by the cell of three men and two women, and rented a green lorry on the Continent. German police said he was the owner of a white Rover car with a damaged rear light, noticed by neighbours at the house in Sandhatten.
Police said residents living near the holiday home hired by the terrorist group had noticed several vehicles there. These included the Ford Transit van used to carry out the attack, which caused damage but no injuries on 28 June, the rented green lorry and the white Rover.
The German authorities released a photograph of the man they named as Robert Dickson and photofits of two other people being sought - identified as Beth and Mark, both in their early twenties. Mr Dickson is believed to have headed for Northern Ireland on 1 July.
A German chief inspector investigating the mortar attack on the Osnabruck base yesterday spent a second day in Northern Ireland. She met Royal Ulster Constabulary special branch officers in Portadown, Co Armagh, on Thursday.
A car seized by police in Portadown on Thursday is thought to be connected with inquiries into the Osnabruck attack.