WASHINGTON -- Airport screeners will have to wait a few more weeks before finding out which union will represent them in collective bargaining talks with the government.

After a six-week election, neither of the two unions vying to represent 44,000 workers at the Transportation Security Administration received a majority of votes cast when federal officials tallied the results on Wednesday.

The vote came after TSA head John Pistole agreed in February to grant screeners limited collective bargaining rights for the first time since the agency was formed a decade ago.

The Federal Labor Relations Authority says it will hold a runoff election to decide a winner between the top vote getters. The American Federation of Government Employees received 8,369 votes, or 43 percent, while the National Treasury Employees Union won 8,095 votes, or 41 percent. The choice of no union received 3,111 votes, or 16 percent.

To win outright, one of the unions had to win 50 percent plus one vote of all votes cast.

It was the largest union election for federal workers in history. The screeners who x-ray bags and shuttle airline passengers through rope lines had been among the few federal workers without union rights.

Republicans have objected vigorously to the decision granting collective bargaining rights, arguing that unionizing would jeopardize national security. But Democrats and labor leaders say union rights have not hampered the effectiveness of federal security workers at other agencies. And Pistole says bargaining rights will help improve low morale at the agency.

Pistole's decision allows for negotiation on a narrow range of issues including work shifts, transfers, vacation time and awards, but specifically prohibits bargaining on security-related matters like deployment, job qualifications, testing or discipline. Screeners also would be strictly prohibited from striking or engaging in work slowdowns.

"No matter which union ultimately prevails, we hope and expect that they will join us to further improve TSA's performance of its critical security mission and support our frontline officers as they carry out this mission," Pistole said in a prepared statement.

Leaders of both unions expressed confidence they would win the runoff, which is likely to begin next month and could be concluded in July. AFGE is the nation's largest federal employee union, representing 625,000 workers in the federal government and the government of Washington, D.C. NTEU is an independent union that represents 150,000 employees in 31 agencies and departments.