The head of an investor group that’s trying to build a massive development at the Oakland Coliseum complex said Thursday the Raiders football team could leave Oakland if Alameda County officials don’t get involved in negotiations soon.

Speaking to the West Oakland Commerce Association, Floyd Kephart, the lead executive of New City Development LLC, said city of Oakland officials have been “very straightforward” in working on the Coliseum City project but he said, “We don’t have that same thing from Alameda County.”

Kephart said, “I’m confused on a daily basis” about the county’s unwillingness to sign off on the proposed development so far.

UPDATE 9:01 p.m. Raiders charge toward SoCal with stadium plan.

Kephart, the chairman of the board of Renaissance Companies, a San Diego firm that advises hedge funds, private equity groups and financial institutions, said a development plan for the Coliseum site “has to be done in the next few months or the Raiders will leave.”

He said, “The Raiders are doing all they can” to make the project, which includes a new football stadium for the team, happen and “it would be tragic if they leave.” Kephart told the business group that Raiders owner Mark Davis called Alameda County Board of Supervisors President Scott Haggerty on Wednesday and “asked him to push this along.”

But Haggerty said he’s already talking to Davis on a weekly basis and county officials are committed to retaining all of Oakland’s pro sports teams, which are the Raiders, the A’s baseball team and the Golden State Warriors basketball team.

Alameda County’s participation is a key component for the $2 billion-plus Coliseum City project because the county and the city own about two-thirds of the 200 acres at the Coliseum site where the development is proposed. Plans call for at least one new sports stadium at the site plus housing, retail stores, hotels and housing.

Kephart said the Coliseum City project “could be the vibrant urban center that everyone envisions and include 5,700 residential units and 475,000 square feet of retail space. But he said the development “is hung up on the city and county coming together on land.”

About $106 million of outstanding debt remains on the current O.co Coliseum, which is used by both the Raiders and the A’s baseball team, but any deal to pay off the debt would depend on the county’s participation. Haggerty said, “There are implications for the taxpayers” and county and city officials need to perform “due diligence” before they sign off on the project.

The Oakland City Council recently granted a 90-day extension to New City Development on its exclusive negotiating agreement with the city to develop the project. Oakland Mayor Libby Schaaf said she is proposing that the agreement be amended to also include the county so the two government entities can work together:

“The county has got to be at the table with the city.”

Schaaf said the project hasn’t moved forward “as quickly as any of us would have liked” but she understands that the county needs time to research the development proposal before it participates in the process.

The city envisions up to three new sports venues at the site: a new football stadium for the Raiders, a new baseball park for the A’s and a new arena for Warriors basketball games and other events. But so far only the Raiders have expressed strong interest in participating in the project and the Warriors currently plan on moving to a new arena in San Francisco in the next few years.

Kephart said if the A’s commit by next year to build a new baseball stadium at the Coliseum site it would be possible to build both a new football stadium and a baseball stadium at the site in the next five years. “You could do two stadiums in five years,” he said. Kephart also said, “There is enormous support in the community” for the Coliseum City project.