Updated 3:10 p.m.

The largest federal union organizing election is not over yet.

The Federal Labor Relations Authority declared Wednesday that a runoff is necessary because neither the American Federation of Government Employees nor the National Treasury Employees Union received a majority of the 19,587 votes cast by transportation security officers during a six-week election period that ended Tuesday.

AFGE received 8,369 and NTEU won 8,095 votes. The “no union” option was chosen by 3,111 voters.

In the runoff, the “no union” option will drop off the ballot, so the two unions will compete only with each other.

Although the two largest federal unions campaigned vigorously during the six weeks of voting that ended Tuesday, they have been recruiting members and seeking the support of the officers for years.

Activity started heating up in November when the labor relations authority ruled that the officers, who screen people and luggage at the nation’s airports, had the right to vote on union representation, a right that had been denied them to that point.

In making that ruling, the authority rejected claims from conservative Republicans in Congress and the National Right to Work Legal Defense Foundation, which argued that allowing unionization among the Transportation Security Administration employees would, in the words of the Foundation “post an unacceptable threat to national security.”

The FLRA ruling did not grant the screeners the right to bargain as a group. That was done in February when TSA Administrator John Pistole granted limited collective bargaining rights.