JEFFERSON CITY (AP) – Missouri lawmakers are vowing an aggressive investigation into why Gov. Jay Nixon did not use National Guard troops to prevent angry crowds from burning and looting businesses in Ferguson when a grand jury decision was announced in the police shooting of Michael Brown.

A House and Senate investigative committee met for the first time Thursday and pledged to use subpoenas, if necessary, to compel members of Nixon’s administration to testify and turn over documents.

A grand jury decided Nov. 24 not to indict Ferguson police officer Darren Wilson, who is white, for fatally shooting the black 18-year-old in August. After the announcement, some protesters set fire to businesses and vehicles.

Nixon had activated the National Guard before the announcement. But troops were not stationed in front of the Ferguson businesses.