MEXICO CITY — President Andrés Manuel López Obrador of Mexico created a truth commission on Monday to re-examine the case of 43 students whose disappearance in 2014, still unsolved after a disputed investigation, has come to represent the tens of thousands of Mexicans who have vanished in more than a decade of the drug war.

Just two days after taking over as Mexico’s leader, Mr. López Obrador set a new tone for the government, pledging to deliver justice to victims of violence and corruption.

“I assure you there won’t be impunity in this sad and painful case,” said Mr. López Obrador, flanked by two of the missing students’ parents. The students’ relatives, many from rural communities in Mexico’s poorest states, sat in the front row of the president’s first public event at the National Palace, holding large images of their missing sons.

Alejandro Encinas, the incoming deputy interior minister for human rights, will head the commission. It will start a new investigation under a special prosecutor’s office and will aim to consider all leads, including those that were ignored or discarded by the former government.