SAO PAULO (Reuters) - Brazil’s securities regulator CVM is investigating the management changes at state-run oil company Petroleo Brasileiro SA, its website showed on Monday, following President Jair Bolsonaro’s announcement last week that he will replace chief executive Roberto Castello Branco. The post on the CVM website gave no details about the investigation, but comes after Petrobras shares plunged more than 20% on Monday following Bolsonaro’s announcement. Under Brazilian securities law, listed companies must communicate important information via securities filings, which did not happen when the president decided to replace Petrobras CEO. He mentioned that in interviews and the nomination of a replacement to current CEO Roberto Castello Branco in this case. Executive changes must be approved by the board of directors, which did not happen here either. Reporting by Aluísio Alves; Writing by Jamie McGeever