In the past couple of weeks we have seen four major terrorist attacks on soft targets in civilian areas: two attacks on mass transit in London; an attack on a shopping mall in Netanya, Israel; and an attack on hotels and resorts in Egypt. The events are a tragic reminder of the dangers we face, the tactics of our enemies and the need for more preparedness.

Every act of terrorism is unforgivable, and any life taken by terrorists is an irreparable loss. This is why we have gone to great lengths as a nation since Sept. 11 to provide law enforcement and intelligence personnel with enhanced tools and information to better identify, track and apprehend terrorists before they are able to strike. We have also made significant progress since last year's bombings in Madrid in securing our mass transit systems.

State and local authorities have received more than $8 billion in Homeland Security Department grants that can be used for mass transit security, and President Bush has proposed an additional $2.4 billion in his 2006 budget. The federal government has chipped in more than $255 million for state and local transit authorities to increase protection through hardening of assets, greater police presence during high alerts, additional detection and surveillance equipment, increased inspections, and expanded use of explosives-sniffing dog teams.

These expenditures reflect our commitment to protecting all of our infrastructure from terrorist attack. Of course, the way in which we do so depends on the nature of the system that we are protecting. Mass transit is an open, accessible and efficient system across a broad geographic area. By contrast, our aviation system is a closed system that can be tightly monitored at controlled checkpoints. An airport-style security system would be poorly suited to local mass transit systems because long delays would interrupt fluidity and convenience. We cannot destroy with draconian security measures the very thing we are trying to protect.

While securing our subways, buses and rail systems remains a critical effort, it has never been solely or even primarily a federal effort. State and local officials rightly control almost all of the "boots on the ground" used to provide security for mass transit. Indeed, these highly trained local law enforcement personnel understand the unique design characteristics and security challenges of their hometown subway, light rail, bus and ferry systems better than anyone.

To make mass transit systems more secure, we need an effective partnership between federal, state and local officials that builds on the strengths and resources each can offer -- and that reflects the individual architecture of each local system. Here are three suggestions for strengthening the federal role in mass transit security while preserving our partnership with our state and local counterparts:

* We must use real-time intelligence and the experience gained from London and Madrid to improve our protective measures. Steps taken to identify and intercept sleeper cells and to secure our borders strengthen mass transit security and our entire infrastructure.

* We should not automatically focus all our resources and attention on subway trains and buses just because the terrorists in London targeted subway trains and buses. The attack on hotels and resorts in Egypt shows that terrorists are interested in other "soft" targets as well. Even within the mass transit sector, we need to think about other elements of the system, such as tunnels and ferries.

For this reason, the Bush administration has proposed $600 million in Targeted Infrastructure Protection Program (TIPP) grants, which will give our states, cities and counties discretion in how they provide protection for mass transit and other critical infrastructure. As Congress debates homeland security funding, it should build on the model of the TIPP grants and give maximum flexibility to allocate grants according to risk, rather than dividing money into categories based on the latest attack.

* Finally, catastrophic attacks on mass transit infrastructure have the potential to kill thousands -- and preventing them must be a primary focus of federal resources. A chemical, radiological or biological attack on mass transit could cripple the system. We must expedite research and development of more advanced equipment to detect chemical, radiological and biological substances so we can stay ahead of the terrorists' intent to unleash mass casualty attacks.

No amount of security can guarantee 100 percent safety, which is why we are applying a risk-based approach to our homeland security efforts. But working with our state and local partners, we can build on each other's strengths to make our mass transit systems as safe as possible.

The writer is the U.S. secretary of homeland security.