SEOUL-- South Korean President Moon Jae-in on Tuesday ordered a thorough investigation into the unauthorized delivery of four more mobile launchers of the U.S. missile shield to the country.

Yoon Young-chan, chief presidential press secretary, told a press briefing that President Moon said it was "very shocking" after he was briefed on the unauthorized transportation.

Top presidential security advisor Chung Eui-yong reported to Moon that four more THAAD mobile launchers were secretly transported to South Korea in addition to two launchers already installed.

Moon ordered the security advisor and the senior presidential secretary for civil affairs to thoroughly look into the secret delivery.

About two weeks before the May 9 presidential by-election, two mobile launchers and a radar for the U.S. Terminal High Altitude Area Defense (THAAD) missile interception system were transported in the middle of night to a golf course at Soseong-ri village in Seongju county, North Gyeongsang province.

THAAD is composed of six mobile launchers, 48 interceptors, a AN/TPY-2 radar and a fire and control unit.

The clandestine transportation raised speculation that it was aimed to politicize security issues during the election campaign period, which tended to benefit conservative candidates in past elections.

Earlier in the day, President Moon gave a phone call to Defense Minister Han Min-koo, who was appointed by ousted President Park Geun-hye, to verify the secret delivery of four more launchers, according to the presidential Blue House.