Washington (CNN) Special counsel Robert Mueller's team has expressed interest in the Trump campaign's relationship with the National Rifle Association during the 2016 campaign.

"When I was interviewed by the special counsel's office, I was asked about the Trump campaign and our dealings with the NRA," Sam Nunberg, a former Trump campaign aide, told CNN.

The special counsel's team was curious to learn more about how Donald Trump and his operatives first formed a relationship with the NRA and how Trump wound up speaking at the group's annual meeting in 2015, just months before announcing his presidential bid, Nunberg said.

Nunberg's interview with Mueller's team in February 2018 offers the first indication that the special counsel has been probing the Trump campaign's ties to the powerful gun-rights group. As recently as about a month ago, Mueller's investigators were still raising questions about the relationship between the campaign and the gun group, CNN has learned.

A spokesman for the special counsel's office declined to comment.