The nation's top cop vowed Tuesday to "do all that we can" to combat what he called a "simply unacceptable" number of local and federal law enforcement officers killed in the line of duty.

In brief remarks before a closed-door meeting with police chiefs from around the country, Attorney General Eric Holder said nothing is "off the table," including an Obama administration push for Congress to approve new legislation.

"In recent months, we have really seen a dramatic rise in the number of police officers and other law enforcement agents who have been killed in the line of duty," Holder said.

After a two-year decline in law enforcement deaths, that number "spiked" last year, with 20 officers killed by guns or assault in the first three months alone, according to Holder. In the same time period this year, 27 officers have been killed by guns or assault, including Immigration and Customs Enforcement agent Jaime Zapata in Mexico, Deputy U.S. Marshal John Brookman Perry in Missouri, and Deputy U.S. Marshal Derek Hotsinpiller in West Virginia.

"These numbers concern me, as they should all Americans," Holder wrote Tuesday in a letter to Justice Department officials across the country. "We rely on our law enforcement officers to protect our lives and property and we must do all that we can to make sure they are protected while on duty."

Holder issued that letter after meeting with 20 state and local police chiefs -- from California and Minnesota to North Carolina and Florida -- and several federal officials, including FBI Director Robert Mueller and U.S. Marshals Service Director Stacia Hylton.

In the meeting "a lot of different ideas" were "put on the table," including legislative measures and steps that could be taken with existing law through increased federal prosecution, according to police officials in attendance. Holder's letter was specifically addressed to U.S. attorneys, the Justice Department's lead prosecutors in federal districts, directing them to meet with local law enforcement and, among other things, identify the "worst of the worst" offenders in their districts, discuss possible "stiffer" prosecutions, and ensure that local law enforcement are "fully informed about the resources that the [Justice] Department makes available to help protect officers."

One of the police officials at Tuesday's meeting said the larger discussion "shouldn't just get framed around, 'What can we do to make police officers safer in America?'"

"The number of officers killed is bucking the national trend. While homicides in our communities are going down, officer deaths are going up," Baltimore Police Department commissioner Fred Bealefeld said after the meeting. "[But] these same guys that are taking shots at cops, they're taking shots at people on the street. And so it ... should be, 'What can we do to make Americans safer?' And if we get to those things and using the laws that are on the book, leveraging federal prosecutions, we can start getting some of these really [dangerous] offenders put away and off our streets. ... It makes all of us safer."

Montgomery County (Md.) Police chief Tom Manger agreed, calling Tuesday's meeting "very valuable" and praising Holder's "willingness" to discuss the issue.

"He has an opportunity from his bully pulpit to say some things in support of law enforcement around the country and talk about officer safety issues," Manger said.

The Justice Department invited the National Rifle Association to attend Tuesday's meeting, but the group declined the invitation, telling Reuters that such moves were "a transparent attempt to appease the anti-Second Amendment base of President Obama."

Holder seemed to dispute that assertion on Tuesday, saying, "We want to make sure that we have dialogue in addition to everything else, with [the relevant] groups, including the NRA, so that we might find ways in which we can come up with common sense approaches to reducing this level of violence that our law enforcement agents are exposed to."

Bealefeld, the Baltimore Police Department commissioner, acknowledged that "a lot of second amendment issues get pulled in" to the discussion. But he said there is "certainly the question of urgency," noting that he currently has "a young officer that's in shock trauma right now" after being shot on Friday in the line of duty.

"He was actually doing what we're pushing our guys and gals to do, and that is targeting gun offenders across the city," Bealefeld said. "Before the officer could take a couple steps, the man had a handgun out and shot him in the neck. Fortunately, my guy's going to survive. But there are a lot of other chiefs up there that have buried officers in the line of duty in the last few months. And so we can't rely on luck or the grace of God. There are some real affirmative steps we need."

Noting that many of the guns his officers come across are not AK-47s or other high-powered weapons, Bealefeld said the man who shot his officer was arrested on a handgun violation in 2009, but the man only served four months in local prison.

"One of the things I've urged is more federal intervention and prosecution of gun offenders in our jurisdiction," he said. "So again, we're hopeful for what comes out of it."

The chief of police in Washington, D.C, Cathy Lanier, said that even the "best equipment, best training [and] best police officers" can't always prevent "being killed by ambush and gunfire."

"It's tough," she said. "We have a lot of technology and a lot of good equipment, but that can't always keep us safe. The assaults on officers and the officers killed in the line of duty ... is scary."

Possible offenders also have a lot of technology, with the growth of social networking sites creating new types of threats for officers.

In October 2010, local counterterrorism officials in Phoenix issued an alert warning that officers had been "targeted on Facebook."

During an alcohol-related traffic stop the day before, police officers "uncovered a CD containing multiple photographs and names of over 30 Phoenix PD officers and civilian employees," according to the alert.

"All of the names and photographs found on the CD were obtained from Facebook and reveal the identity of several patrol and undercover officers. ...  It is unknown how many more CDs (if any) may be circulating," the alert said, adding that officers "should use good judgment when posting personal information about themselves on social networks."

In his letter to U.S. attorneys, Holder said he is "convinced that we can reduce violent crime and reverse the officer fatality trend that we saw in 2010 and earlier this year."