Seoul Dongbu Detention Center, located in eastern Seoul, where former Defense Minister Kim Yong-hyun is currently imprisoned (Yonhap)

Police retrieved a secure mobile phone used by former Defense Minister Kim Yong-hyun during a raid on the defense ministry headquarters Thursday, officials said.

Police investigators located the phone sent to the presidential office compound to seize evidence related to President Yoon Suk Yeol's short-lived declaration of martial law last week.

The defense ministry building is located in the same compound alongside the presidential office and the Joint Chiefs of Staff (JCS) headquarters.

Yoon and Kim reportedly used secure phones to relay orders to commanders in the field during the six hours that martial law was in effect between Dec. 3 and 4.

Police also raided the Capital Defense Command headquarters and seized material from servers connected to the phones, which are installed with anti-eavesdropping and anti-voice recording software.

The JCS, however, opted to voluntarily submit material, including surveillance camera footage of the command and control center in its building's basement, which was used as the martial law command's situation room before and after Yoon announced the decree.

Yoon reportedly met with military officials at the command and control center shortly after the National Assembly voted to lift his decree at around 1 a.m. on Dec. 4.

Meanwhile, the presidential office was not subject to the raid on Thursday.

The previous day, police sought to search key offices, including the Cabinet conference room, the Presidential Security Service and the bunker room inside the JCS headquarters, but that raid ended with the police being given limited documents from the presidential security service.

Police had tried to confiscate Kim's phone when they searched his office and official residence during a raid on Sunday but were unable to secure it at the time. (Yonhap)