SAN FRANCISCO (CBS SF) — While U.S. Senators Dianne Feinstein (D-California) and Barbara Boxer (D-California)didn’t participate in a gun control filibuster that took place on the Senate floor Wednesday night, they are co-sponsoring an amendment to the Commerce, Justice, Science Appropriations bill to increase background checks in order to keep guns out of the hands of known or suspected terrorists.

The filibuster to force a vote on gun control legislation and the amendment come in the wake of the mass shooting at a gay nightclub in Orlando, Florida that left 49 people dead and 53 wounded.

Connecticut Sen. Chris Murphy (D-Connecticut), who launched the filibuster, is seeking a vote on Feinstein’s legislation that would let the government stop sales of firearms and explosives to suspected terrorists. She initially introduced her amendment in December, a day after a mass shooting in San Bernardino, California, but her proposal was rejected by the Republican-run Senate.

Senator Feinstein released updated data from the Government Accountability Office (GAO) on Wednesday that shows 91 percent of the time, known or suspected terrorists pass a background check to purchase a firearm or explosives.

Feinstein said, “Between February 2004 and December 2015, known or suspected terrorists initiated a background check to purchase a weapon 2,477 times—they successfully passed that check 2,265 times, or 91 percent.”

Diana C. Maurer, director of Homeland Security and Justice Issues at the U.S. Government Accountability Office wrote in a letter to Feinstein in March that of the 2,477 known or suspected terrorists who initiated a background check to buy a firearm, only 212 were denied for sale of a weapon between 2004 and 2015.

The amendment, co-sponsored by Boxer, Feinstein and 25 of their Senate colleagues aims to give the attorney general authority to block the sale of guns or explosives to known or suspected terrorists.

The amendment would also ensure that anyone who has been the subject of a federal terrorism investigation in the past five years would get flagged for further review if they attempt to get a background check to purchase a firearm.

“In light of the worst mass shooting in our nation’s history, the dangerous loophole in our laws that allows known or suspected terrorists to legally purchase guns has again been exposed. We must act to close this loophole,” Feinstein said. “Over the past decade, 91 percent of individuals who are known or suspected terrorists passed background checks, showing this is a pervasive problem. My amendment would simply allow the attorney general to block gun sales to these individuals and closing this loophole is the least we could do to reduce the risk of terrorist attacks in our country. I continue to work with my colleagues in hopes of reaching a compromise that can pass the Senate.”

Senator Murphy said late Wednesday night, as the filibuster was in its 12th hour, that Feinstein’s amendment “addressed the concerns that Republicans have raised” and only stops those on the terrorist watch list and stops the sale of guns to criminals.

The FBI does not currently have the authority to block the sale of a firearm to a person on a terrorist watch list, but the agency is merely notified when that person has applied for a background check through the National Instant Criminal Background Check system. This proposed amendment aims to decrease the risk of terrorism and mass shootings by stopping people who are a known or suspected terrorist.

The marathon filibuster ended after nearly 15 hours, with Senator Murphy announcing that Republican Senate leaders agreed to hold a vote on closing the terrorist watch list gap and expanding background checks for firearm sales.

By Hannah Albarazi – Follow her on Twitter: @hannahalbarazi.