Developer Robert Maguire, the largest commercial building owner in Downtown L.A., has broken ranks with his fellow building owners and agreed to let security guards in his buildings form a union, it was announced Tuesday.

Maguire Properties Inc., a real estate investment trust, has reached an agreement with the Service Employees International Union, to create a new security officer-only union local in Los Angeles.

The breakthrough in SEIU’s three-year quest to unionize security guards at major office buildings in L.A. came after L.A. Mayor Antonio Villaraigosa intervened several weeks ago. Villaraigosa hosted Tuesday’s press conference announcing the deal.

“Today’s agreement was reached after substantial ongoing dialogue with SEIU and final sign off by (SEIU president) Andy Stern and myself,” said Maguire at the press conference. “Mayor Antonio Villaraigosa played a critically important role in getting to this agreement by helping bridge the gap on several key issues

Villaraigosa thanked Maguire for taking the lead in the negotiations and also pointed to Maguire’s role in ending a major janitors’ strike six years ago. “Los Angeles is fortunate to have a business owner who so greatly respects those who work in his buildings,” Villaraigosa said.

The agreement also calls for a jointly-subsidized program to train security guards in emergency preparedness and response. That has been a key demand of city officials and union leaders.

“We believe this training program will be particularly meaningful with positive national implications as it is implemented,” Maguire said.

Even with Tuesday’s announcement, the L.A. City Council is still moving ahead with an ordinance to impose training standards and require minimum levels of benefits for security guards in major L.A. buildings.

The proposed ordinance would impose training requirements for security guards in all major buildings in the city and require that building owners and/or security guard contractors in major buildings provide minimum levels of health benefits to security guards.