WASHINGTON — NASA has pushed back by three months a decision on a new series of contracts to transport cargo to and from the International Space Station, claiming it needs more time to review the proposals it received.

NASA posted a message April 16 on the procurement website for the Commercial Resupply Services (CRS) 2 contract stating that the estimated award date was now Sept. 16. The site had previously listed an award date of June. That message stated that the agency revised the date “due to additional time required to evaluate proposals.”

NASA’s associate administrator for human exploration and operations, William Gerstenmaier, confirmed the delayed award date during a meeting of the National Research Council’s Aeronautics and Space Engineering Board here April 21.

“The selection was expected in June,” he said in a discussion about the CRS-2 contracts. “We just moved that to September. I can’t say much more about this because we’re in blackout,” a reference to the restriction on information the government can provide during an ongoing procurement.

When NASA issued the request for proposals for CRS-2 in September 2014, it anticipated awarding contracts in late April. Shortly after the deadline for proposals passed in early December, NASA revised that date to June. NASA stated it planned to award multiple contracts, each including a minimum of six cargo flights to and from the station.

Five companies have publicly stated that they submitted CRS-2 proposals. Those companies include SpaceX and Orbital ATK, which have the two existing CRS contracts. Boeing and Sierra Nevada Corp. also submitted proposals using vehicles derived from designs developed under NASA’s commercial crew program. Lockheed Martin announced in March it was also competing for CRS-2 using a system featuring a reusable tug and expendable cargo module.

SpaceX launched the sixth of its 12 original CRS missions April 14, with the spacecraft berthing with the ISS April 17. The other six flights are scheduled for launch between June 2015 and August 2016, according to a schedule Gerstenmaier showed in his presentation.

Orbital ATK, which suffered a failure of its Antares launch vehicle on its third CRS mission in October, plans to return to flight in the fourth quarter of this year by launching its Cygnus cargo spacecraft on a United Launch Alliance Atlas 5. The company plans to resume Cygnus launches on a new version of the Antares rocket in early 2016.

Orbital’s contract originally covered eight missions, including last October’s failure, but the company and NASA agreed it could fulfill its cargo requirements with seven missions.

NASA recently extended both companies’ CRS contracts, adding three additional cargo missions to SpaceX’s award and one to Orbital’s. Those launches have not been scheduled but would likely extend into 2017.

Gerstenmaier said it’s possible NASA could add more cargo missions to those CRS contracts, although it may require extending the contracts’ period of performance. “We’ll go through that and see what happens with the overall CRS-2 contract activity and figure out what we need to do,” he said.