Las Vegas Metropolitan Police Sheriff Joseph Lombardo holds a news conference on Tuesday afternoon, Oct. 3, 2017. (Screen grab from C-SPAN)

(CNSNews.com) - The 64-year-old man who opened fire on Las Vegas concert-goers Sunday night installed bump stocks on 12 of the semi-automatic rifles found in his hotel room, an ATF agent told "CBS This Morning."

Bump stocks are among the devices, legal at the moment, that allow a semi-automatic weapon to fire continuously, but weapons equipped with bump stocks do not fall into the machine gun category.

"A semi-automatic weapon with a bump-fire stock on it is not an illegal machine gun," Jill Snyder, a special agent in charge at the Bureau of Alcohol, Tobacco, Firearms and Explosives, told CBS's Norah O'Donnell.

On Tuesday afternoon, Las Vegas Metropolitan Police Sheriff Joseph Lombardo told reporters that the gunman had modified his weapons to make them fire faster.

"ATF is participating in that evaluation," he told a news conference. "I can't give you an answer on whether any of them are automatic or not, but we are aware of a device called a bump stock and that enables an individual to speed up the discharge of ammunition. I don't want to give you any more details than that, but in partnership with the FBI, the ATF, they are sending those weapons back East to the FBI crime lab for further evaluation," Lombardo said.

ATF Special Agent Snyder was more forthcoming with information about the gunman's firearms:

"From October 2016 to September 28, 2017, he purchased 33 firearms, majority of them rifles," Snyder told CBS. "We wouldn't get notified of the purchases of the rifles, we would only get notified if there was a multiple sale, which would be two or more handguns in an individual purchase," she added.

Snyder also told CBS that the gunman used magazines holding 60 to 100 rounds each.

Sen. Dianne Feinstein (D-Calif.) on Tuesday said she plans to introduce legislation to "close the automatic weapon loophole."

The senator issued a statement noting that the sale and manufacture of automatic weapons have been illegal since 1986, and automatic weapons produced before 1986 are regulated and tracked by the ATF:

“Despite this, individuals are able to purchase bump fire stocks for less than $200 and easily convert a semi-automatic weapon into a firearm that can shoot between 400 and 800 rounds per minute and inflict absolute carnage," Feinstein wrote. "The Las Vegas shooter appears to have modified at least one of his weapons in this way.

“A ban on bump fire stocks was included in my 2013 assault weapons bill, and I’m looking at how best to proceed with legislation to finally close this loophole. This is the least we should do in the wake of the deadliest mass shooting in U.S. history. It should be our highest priority.”

In other news, the gunman's 62-year-old girlfriend has returned to the United States, arriving in Los Angeles overnight from the Philippines. Lombardo on Tuesday called her a "person of interest" in the case, and the FBI is believed to be interviewing her now.