Explainer-What you need to know about investigations faced by South Korea's former president Yoon

SEOUL (Reuters) - Since South Korea's Constitutional Court ousted President Yoon Suk Yeol this month, he has lost presidential immunity and is vulnerable to additional charges on top of an ongoing criminal trial concerning his martial law declaration on December 3.

Yoon is on trial over insurrection charges, along with a former cabinet minister, several military commanders and police officials.

Here is what we know about the criminal investigations and trial so far: 

WHO IS FACING CHARGES? 

Yoon attended the first arguments of his criminal trial on Monday where prosecutors accused him of lacking the legal grounds to declare martial law and ordering troops to arrest lawmakers.

Yoon, who has been charged with masterminding an insurrection, denied all the charges and argued that his martial law declaration, which lasted about six hours, did not amount to insurrection.

As a defendant in a criminal trial, Yoon was required to attend Monday's arguments heard at the Seoul Central District Court.

His former defence minister, Kim Yong-hyun, has been arrested and indicted over alleged insurrection and abuse of power. Chiefs of the Capital Defence Command and the Defence Counterintelligence Command have also been indicted.

Others involved in the case include army chief Park An-su, who was named martial law commander and has been indicted. The army's Special Warfare Command as well as several other military generals and senior police officials, including the national and Seoul police chiefs, are also accused of playing a role in the martial law plan. 

Yoon, Kim, Lee, the police chiefs and more than 10 military commanders have been banned from leaving the country. 

Yoon became the first sitting president to be arrested on January 15 after a dramatic showdown with law enforcement officials.

He was released from jail on March 8 after a court cancelled his arrest warrant, citing the timing of the indictment and questions about the legality of the investigation.

WHAT CHARGES ARE SOUGHT?  

In what had been the first-ever criminal prosecution of an incumbent leader, Yoon was indicted in January on charges of leading insurrection.

Other officials also face charges of insurrection, abuse of authority and obstructing other people from exercising their rights. 

South Korean investigating authorities pursued the charge against Yoon because insurrection is one of the few criminal charges from which a sitting South Korean president does not have immunity.

However, with Yoon having lost immunity, he is now open to other potential charges prosecutors may bring against him.

If convicted, leading an insurrection is punishable with life imprisonment with or without labour, or death.

For those who engaged in activities key to the insurrection, punishment could range from death to life imprisonment to imprisonment without labour for at least five years. 

South Korea last handed down a death sentence in 2016, but has not executed anyone since 1997.

People who joined in the plot or violence face imprisonment, with or without labour, for less than five years.

(Reporting by Ju-min Park, Hyonhee Shin and Joyce Lee; Editing by Ed Davies and Gerry Doyle)