The decision to reopen public comments came after an independent study questioned the information used to make the decision, the (Palm Springs, Calif.) Sun reported Friday.
The government plan would have allowed gray wolves in the lower 48 United States to be hunted or killed.
They have been under federal protection since 1967. The Mexican gray wolf, found only in the Southwest, retains protection.
The National Center for Ecological Analysis and Synthesis, which conducted the study, charged the wildlife service's delisting proposal was "strongly dependent on a single publication, which was found to be preliminary and not widely accepted by the scientific community."
The agency had asked the center, part of the University of California Santa Barbara, for the objective study after receiving criticism from conservationists and scientists whose work was used to develop the new rules.
The study's findings prompted FWS to reopen public comment for another 45 days. The initial comment period ended in December.
The latest analysis reaches many of the same conclusions as a previous analysis, which was rejected after claims FWS tried to remove scientists who had signed a letter expressing concerns about delisting, the Los Angeles Times reported.
A final decision by FWS is expected in late 2015.
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