As part of the investigation, searches were carried out on Thursday at the offices of the business group's president and vice-president, as well as at Masiulis' home, car and office and at the Liberal Movement's headquarters.
"Information available in the pre-trial investigation gives us grounds to suspect that representatives of a business concern might have handed a bribe of a very large scale (around €100,000) to Seimas member Eligijus Masiulis for the political party to act in their favour," the STT said in a press release.
"I can confirm this," STT spokesman Ruslanas Gorbačiovas told BNS when asked if the business group in question was MG Baltic.
The law enforcement body did not disclose the status of MG Baltic's top executives in the pre-trial investigation.
A spokesman of MG Baltic confirmed on Thursday that searches had been carried out at the business group's premises, but did not comment on bribery suspicions against Masiulis.
Romanas Raulynaitis, a member of the management board at MG Baltic, confirmed to BNS that the offices of Darius Mockus, the group's vice-president, and Raimondas Kurlianskis, vice-president and director of Alfa Media, the owner of the Alfa.lt news website, had been searched.
The STT said that a decision had been made to question Masiulis as a special witness.
Members of the Seimas enjoy immunity from criminal prosecution, which can only be lifted by the parliament.
The pre-trial investigation over possible bribery, graft and large-scale influence peddling is controlled and led by a group of prosecutors from the Organized Crime and Corruption Investigation Department at the Prosecutor General's Office.
MG Baltic is one of Lithuania's biggest manufacturing, trade, real estate and media groups. It employed a workforce of over 3,000 in 2014.
The group's companies include the alcoholic drink producers and distributors Stumbras, Alita and Mineraliniai Vandenys, the TV channel LNK, the news website Alfa.lt, the real estate companies Mitnija and MG Valda, the clothing retailer Apranga, and the logistics company Tromina.
Liberals pledge to cooperate with investigators
Gintaras Steponavičius, deputy chairman of the Liberal Movement, said the party was ready to cooperate with investigators during a press conference at the parliament.
"The Liberal Movement has always been a proponent of transparency in politics. Therefore, we will provide all the necessary assistance to law-enforcement bodies to help them complete their investigation as quickly as possible," Steponavičius said in a press release.
He said he had not discussed the matter with Masiulis yet and cautioned against jumping to conclusions. He added that the party was calling a meeting to address the issue.
Liberal MEP Antanas Guogas said in a Facebook post the news was "a bolt from the blue". "I am shocked, because suspicions alone leave very many people disappointed. I trust Lithuanian law-enforcement and believe it will soon discover what is really going on," he wrote.
Algirdas Sysas, deputy chairman of the ruling Social Democratic Party, said that the STT's investigations often lead nowhere and that the service becomes more active before every elections.
Menawhile conservative leader Gabrielius Landsbergis said he was "shocked" by allegations against the liberals, who are thought of as conservative allies on many issues. He added, however, that he did not think law-enforcement institutions were pursuing the investigations because of the elections.
Lithuania's President Dalia Grybauskaitė has refused to comment on the investigation into the suspected bribery.
"I do not want to speculate. I cannot comment on a pre-trial investigation, and when it comes to its impact on elections – this is a decision to be made by the people," Grybauskaite told journalists on Thursday.
Thursday afternoon, Masiulis briefly spoke to reporters upon leaving his home.
"As a person, I am deeply shaken. Legal proceedings are underway and, unfortunately, I cannot comment. But I do hope that I will be able to present my official position and assessment," Masiulis said.
An aviation bomb was found and later neutralized in the Zverynas area of Vilnius on Thursday night.
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