DETROIT (WWJ) – Delta Air Lines Inc. now faces a second investigation over its handling of another union election.

The federal National Mediation Board told the airline Monday that it would take a closer look at whether elections for three groups of workers were tainted.

Pilots were the only large unionized group at Delta when it bought Northwest Airlines in 2008.  The purchase triggered elections to see whether the unions who had represented Northwest workers would represent the combined groups.

The union has complained that Delta his improperly interfered in those proceedings.