T'way Air Co., a South Korean low-cost carrier, was under investigation by the government over the suspected use of unauthorized aircraft parts, according to industry sources Friday. According to the sources, the Ministry of Land, Infrastructure and Transport recently discovered evidence of T'way Air having used unauthorized parts for its in-aircraft heating and cooling systems during a special investigation. The ministry has carried out the investigation since December in response to a notable increase in flight cancellations and delays by T'way Air last year. Authorities plan to decide on administrative measures against T'way Air after concluding the investigation. "We are comprehensively examining the entire maintenance system, including whether unauthorized parts were used," a ministry official said. T'way Air is set to take over four routes connecting South Korea with major European cities -- Frankfurt, Paris, Rome and Barcelona -- from industry leader Korean Air Co., as part of conditions set by t he European Union over Korean Air's envisioned merger with No. 2 player Asiana Airlines Inc. T'way Air plans to begin operating flights to Paris in June and gradually expand to the other three routes.