Lawmakers were shown a gruesome video depicting dozens of people killed by nerve gas as part of a classified, closed-door briefing Thursday laying out the Obama administration's case for action against Syrian President Bashar Assad.

Senate Intelligence Committee Chairwoman Dianne Feinstein, D-Calif., said the images-- a grim, "CIA meets CSI" presentation aimed to convince, beyond a shadow of a doubt, that Assad bloodlessly killed more than 1,400 people, including over 400 children -- is what other members of Congress need to see if they're to be persuaded to vote to authorize strikes designed to punish Assad and deter him from using chemical weapons again.

Senator Dianne Feinstein tells reporters Thursday that while some lawmakers may be undecided on the best course of action in Syria, there will ultimately be a "moment of truth" when a vote occurs.

"It's horrendous," Feinstein said. She's asked the CIA to prepare a DVD that would include "instances of evidence, largely victims," that could possibly be distributed to members of the U.S. House.

The major Aug. 21 attack outside Damsacus came, Feinstein said members have been told, after between 11 and 14 smaller chemical attacks designed to test the gas.

"It's so grim and ghoulish. And I think -- when people hear nerve gas, it has no picture to it," said Sen. Barbara Mikulski, D-Md., who also attended the classified briefing. "But when you see that it is grim and ghoulish...I think that a sampling for our members to get the reality of this."

Both Mikulski and Feinstein said they left convinced that Assad had used chemical weapons -- and better understood why poisonous gas is outlawed in war.

"It's enough for me," Feinstein said when asked if the evidence convinced her that Assad carried out the attacks. "I think the prohibition on chemical weapons is well founded and after you watch exactly what happens you can see why so."

"I am convinced that these weapons were used," Mikulski said, who described Thursday's briefing as "CIA meets CSI, the forensics."

Mikulski said she is still making up her mind about whether to vote to authorize strikes; she voted against going into Iraq.

The briefing for the Intelligence Committee -- whose members are typically authorized, and sometimes required, to have more access to classified information than other members of Congress -- is part of an all-out push by the Obama administration convince Congress to authorize striking Syria.

Across the nation, taxpayers said they're wary of entering another war and think the funds could be spent better elsewhere. NBC's Andrea Mitchell reports.

A significant part of that is convincing members that the attack took place and that Assad is responsible. That's partly because so many members still feel burned by the conflict in Iraq -- they voted to authorize going in based on intelligence that later turned out to be wrong.

But the Ingelligence Committee -- and its companion panel in the House of Representatives -- usually has more extensive briefings than rank-and-file members. That's why Feinstein suggested that more members might need to see a DVD.

"He's got to understand, Assad, that there is a penalty for this, otherwise you say to everyone there, whoever it is, there is no penalty if you go ahead use this terrible nerve agent," Feinstein said.

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