SOFIA, Oct 7 (Reuters) - Three senior foreign employees of Bulgaria"s biggest weapons maker Arsenal have been arrested on suspicion of being involved in the disappearance of sensitive products and documents from the company"s plant, the interior ministry said on Thursday. Law enforcement authorities launched an investigation this month after the private arms company reported that specific products and sensitive documentation which could be of interest to local and foreign competitors had gone missing. Police officers established that two Lithuanian nationals and a Russian national, who were implementing innovative technologies at the plant, had been on unpaid leave until Sept. 17 but had not returned to their jobs. The three, however, had been in the central city of Kazanlak where the plant is situated on Oct. 2, the day before the incident was reported, the ministry said. They were detained at the border with Greece on Oct. 5. Police officers found documents and other objects in their vehicles which they believe are directly linked to the probe at the Arsenal weapons plant, the ministry said in a statement. The three are expected to be transported to the regional police office in the city of Kazanlak to face charges. Arsenal produces and exports small arms and artillery systems as well as munitions among other products.