Who is the highest authority?

The speaker constituted a parliamentary committee to investigate the alleged corruption, misappropriation and financial irregularities of the former speaker, his deputy. and chief whip. The speaker did not act alone, he was authorised to make the committee by the House. After a long tireless investigation, the committee has submitted the findings of their investigation before parliament. The investigation has reportedly found proof and hard evidence of corruption and misuse of power by all three during their office.
The former speaker deputy speaker and chief whip, now popularly known as the "trio" did not accept the recommendations of the committee saying that this committee was constituted without lawful authority and they are not compelled to accept it. All of them have also denied the charges and blamed the government saying that this government has done this just to harass them and their party politically. Their party, BNP, has officially challenged the legality of the committee and rejected the committee's report.
So far we know, parliament is the highest authority in any democratic country. The people of the country send their representatives through countrywide election to this parliament to run the government and make laws for them. According to the above "trio" and their party, parliament has no jurisdiction to investigate irregularities of any individual or individuals, then who have the authority and jurisdiction to investigate such matters?