ALBUQUERQUE, N.M. — The agency that oversees the nation’s nuclear weapons stockpile said Monday that further study is needed to determine the best option for the United States as it looks to ramp up production of the plutonium cores that trigger nuclear warheads.

A team of engineering experts from within the National Nuclear Security Administration as well as outside professionals will be considering two options that were identified as part of an earlier review that looked at the most efficient and cost effective means for making the plutonium pits.

Agency spokeswoman Lindsey Geisler told The Associated Press that the options include leaving the work to technicians at Los Alamos National Laboratory in northern New Mexico or moving it to the U.S. Energy Department’s Savannah River Site in South Carolina.

It’s not clear how long the extra analysis will take, but Geisler said new pits must be made “to ensure that U.S. nuclear forces are modern, robust, flexible, resilient, ready and appropriately tailored to deter 21st-century threats and reassure America’s allies.”

The initial report on the risks and capabilities of Los Alamos and the other sites was due over the summer but nothing has been made public other than a redacted summary sheet obtained by a watchdog group in the wake of recent congressional briefings.

The summary suggests that out of five potential sites, it would be most costly — possibly as much as $7.5 billion — to continue making plutonium pits at Los Alamos and that it could be 2038 before the lab would be able to meet production goals.

No new pits have been made since 2011. The Energy Department wants to ramp up production to 80 pits a year by 2030.

Members of New Mexico’s congressional delegation suggest the nuclear agency’s evaluation process was flawed from the beginning. Others are voicing concerns given Los Alamos’ safety record and questions about necessary infrastructure.

The birthplace of the atomic bomb, Los Alamos has for years been the only nuclear weapons site in the U.S. capable of building plutonium pits once the Rocky Flats facility in Colorado closed in 1989. However, work at Los Alamos was stalled by a series of mistakes, and criticisms of the lab have been mounting in the wake of incidents in which nuclear materials were mishandled.

The lab took aim at its critics earlier this year, saying in an internal memo that operations at its plutonium facility and its safety programs have successfully undergone more than a dozen independent external reviews and that it’s close to being fully operational after safety problems forced work to be suspended in 2013.

U.S. Sens. Tom Udall and Martin Heinrich and Congressman Ben Ray Lujan, all New Mexico Democrats, are pushing for the initial report on pit production to be released.

“It’s hard to see how NNSA could justify uprooting and recreating the mission somewhere else will save time and money,” they said in a joint statement.

They argued that the Pentagon, Nuclear Weapons Council and Armed Services committees in Congress have all supported production at Los Alamos.

Greg Mello with the Los Alamos Study Group said the nuclear agency should consider deferring decisions about where, how, whether and when to expand pit production until more is understood about the costs, risks and necessity.