After a day of questioning, prosecutors sent professor Song Doo-yul back to his hotel late Friday night. The South Korean-born German professor was interrogated about his alleged connection with North Korea. An official at the Seoul District Public Prosecutors' Office said the investigation focused on whether he had served on the Politburo of North Korean Workers' Party, how much money he had received from the North and for what purpose. The prosecution plans to summon Professor Song again next Monday for further questioning.During the interrogation, Song allegedly confessed to joining North Korea's ruling Workers' Party and its powerful Politburo.But at a nationally televised news conference on Thursday, Song denied most of the charges. He only adimitted that he joined the communist party in 19-73 in what he said was a "necessary step" to enter the North.The 59-year-old philosophy professor at Muenster University in Germany returned to his native homeland last week after 37 years in exile to face investigation by South Korean officials.The prosecution extended the ban on Song from leaving the country for an additional one month. Prosecutors also plan to file for Song's indictment if the charges leveled against him are confirmed. Meanwhile, South Korea's Ambassador to Germany said the German government wants South Korea to guarantee the access to lawyer and other consular rights guaranteed under international practices in Song's investigations.