When a convicted sex offender is able to live invisibly within a community, no one can be sure they won't hurt someone again.

Such was the case in two separate cases in Florida in recent weeks. In both cases, police say, convicted sex offenders confessed to brutally murdering young girls.

So last week, members of Congress introduced legislation to force states to keep better track of convicted sex offenders and add tougher penalties for those who don't comply with the system.

The bill, named the Jessica Lunsford Act after one of the girls who was murdered, requires states to keep tabs on sex offenders by randomly sending out semiannual mailings to them. The current mailings are sent out once per year and require offenders to report their home address.

The act also would add stricter penalties, including steep fines, to those who don't tell police where they're living. Those who continue to ignore the system could be required to wear an electronic monitor on their ankle.

Although it's a good start, and an admittance that the current system is flawed, the proposed bill is weak.

Convicted sex offenders should be monitored on the federal level in addition to being tracked by the state. Federal legislation also should have a focus on treatment and rehabilitation for offenders who aren't incarcerated.

Sex offenders should be required to check in with a local agency once per month, and in addition to reporting where they live, also should be required to list their employer in case they can't be found at home.

Now we're not necessarily concerned about tracking every move of someone who falls under the "convicted sex offender" category because they were caught urinating in public a few times.

But the legal improvements would be a way to ensure that pedophiles not in prison can't move to another state and begin roaming elementary school playgrounds while local police and families are none the wiser.

The system as it stands is almost ineffective. Law-enforcement agencies can lose track of convicted sex offenders who don't like being tracked.

But by committing a sex crime, one should lose some rights - just as the human rights have been stolen from their victims.

Until each state has an effective way to keep track of convicted sex offenders, the federal government should ensure someone is watching them.