In July 1993, a deranged gunman entered the office building at 101 California St. in San Francisco and moved floor to floor, shooting whomever he saw. He killed eight before taking his own life.

Nearly two decades later, a similarly armed gunman entered a movie theater in Aurora, Colo., shooting 70 people, of whom 12 have died. This was the largest attack of its kind in American history.

Along with the sadness and grief, Americans across the country are asking themselves: Why has so little been done to stop this seemingly endless cycle of violence? Are we helpless in the face of these horrible tragedies, doomed to witness these scenes of carnage again and again?

The answer is as frustrating as the question. Over and over, commonsense measures to protect the American public have been stymied by a powerful gun lobby that has a stranglehold on many in Congress.

The 101 California Street shooting helped galvanize Congress to pass the 1994 crime bill, which included a federal assault weapons ban that I was proud to have authored.

But just a decade later, proponents of the ban were unsuccessful in extending it. That was deeply frustrating.

Contrary to gun-lobby propaganda, the assault weapons ban worked. The 101 California Street attack involved two TEC-9 semiautomatic handguns. The Aurora shooting involved an AR-15-style semiautomatic assault rifle with a 100-round ammunition drum. The manufacture and sale of these weapons, along with the 100-round drum, would have been prohibited under the assault weapons ban.

Who needs these military-style assault weapons? Who needs an ammunition feeding device capable of holding 100 rounds?

These weapons are not for hunting deer - they're for hunting people.

Would these massacres have been prevented if the powerful weapons used were still banned? Maybe not. But it's hard to imagine a gunman using a handgun with a maximum of 10 rounds causing the wholesale slaughter that resulted from a 100-round, semiautomatic assault rifle.

We should be outraged by how easy it is for the perpetrators of these horrific crimes to purchase powerful weapons. As a nation, it's time we engage in a sane conversation about the proliferation of guns in our society.

Let me be clear: If an individual wants to purchase a weapon for hunting or self-defense, I support that right.

But a semiautomatic assault rifle with a 100-round ammo drum - or a handgun with a 30-round magazine like the one used to shoot former Rep. Gabrielle Giffords in Arizona - has but one purpose: to kill as many people as possible in as short a time as possible.

I challenge anyone who claims that prohibiting the purchase of military style assault weapons infringes on American freedoms. No sane person would argue that an individual should be free to own a nuclear weapon - we set limits and we abide by them. And we need to set limits on assault rifles.

Machine guns have been banned in this country for decades. Even as it found an individual right to gun ownership in the Second Amendment, the Supreme Court made the following observation: "Like most rights, the right secured by the Second Amendment is not unlimited," that it is "not a right to keep and carry any weapon whatsoever" and noted "the historical tradition of prohibiting the carrying of 'dangerous and unusual weapons.' " That opinion wasn't written by some wild-eyed liberal - it was written by conservative Justice Antonin Scalia.

I reject the assertion that these massacres are simply a fact of life, that there is nothing we can do about them.

For too long, Washington has bowed to the wishes of the gun lobby, even though numerous surveys show that substantial majorities of gun owners, among many other Americans, support a renewed assault weapons ban. California has banned the sale of assault weapons. President George W. Bush supported the assault weapons ban. Even Mitt Romney signed an assault weapons ban into law as governor of Massachusetts.

To break the stranglehold of the gun lobby, people from across the political spectrum must stand up and say, "No more." No more will we allow these weapons of war to create carnage in our movie theaters, office buildings, schools and playgrounds.

We must not allow another tragedy to occur before we get serious about fixing our nation's gun laws. The assault weapons ban meant fewer Americans were killed. That's a result that Republicans and Democrats alike should embrace.

Assault weapons ban - by the numbers

The federal ban, in effect from 1994 to 2004, worked, as these statistics show.

2/3 the amount by which use of the weapons in crimes dropped during the first nine years of the ban.

7 percent, the decrease in total gun murders in the country during the ban.

450 the number of violent crimes committed involving assault weapons since the law's 2004 expiration.

750 the number of individuals shot using these weapons (of whom 350 were killed).

Sources: Bureau of Alcohol, Tobacco, Firearms and Explosives; Department of Justice; Brady Campaign

Dianne Feinstein is the senior senator from California.