 -- Two cells of terrorists have rocked the United Kingdom, striking in the heart of its two greatest cities. As Americans look on in horror, with sympathy for our British allies, we wonder what lessons we should be taking so that we might avoid such events here. I see six relevant points.

First, some of the weapons being used are commonplace. As in Nice and Berlin, the London attack involved an everyday truck as the weapon, supplemented by kitchen knives. The absence of firearms in the hands of the terrorists is likely due to the United Kingdom’s strict gun laws. Were guns involved, as they easily could be here, the death tolls would have been higher. The point, however, is that terrorists are employing weapons they can easily access and there is little or nothing we can do to stop them from acquiring such items. Cities need to look anew at their streets and venues and consider what can be done to stop a vehicle from plowing into a cluster of civilians on a square, a pedestrian mall, or a row of sidewalk cafes. Simple countermeasures work. At this year’s Boston Marathon, I saw snow plows, dump trucks, and garbage trucks blocking side streets along the race route.

Second, areas around secure venues are often targets. Just as the Manchester bomber placed the lethal device just outside the secure area, other attackers have preyed on lines of people waiting to go through security checks or on facilities such as airline ticket counters just beyond secure zones. Police and security officials need to have both a visible (armed patrols and dogs) and an invisible (undercover behavioral spotters) presence in the “penumbra areas” near secure zones.

Third, watch lists and surveillance systems do not themselves guarantee security. Many of the attackers in Europe (as with incidents in the U.S.) were known to security services and had been placed on watch lists. In the U.K., as here, watch lists have been long, populated by thousands of people whose interest in radical Islamist causes has raised concern. We should not criticize our security services for their inability to do the impossible. Thousands of people cannot be closely monitored in a time sensitive manner, even electronically. Moreover, security services can infrequently detect when a radicalized individual transitions in their own mind into being someone willing to kill and someone willing to die to do it. Changing our laws to reduce our civil liberties and allow more intrusive government surveillance will not solve this problem.

Fourth, the most valuable source of information about the changed intent of a radicalized individual are those who know the person: neighbors, clergy, family, and friends. Warnings from such close associates must be given more serious treatment than apparently has been the case in some of the recent incidents. Security officials need to know and to brief the communities on the known “tells” that indicate someone has moved from simply being radicalized to being a killer in waiting.

Fifth, the willingness of those around radicalized individuals to warn when they see the telltale signs of changed behavior is dependent in part upon good relations between communities and security officials and the government in general. Creating such a positive relationship comes from a perception that the government respects, protects, and assists individuals and communities. Sharing a Ramadan Iftar dinner is not just being a good neighbor, it is part of creating a feeling of mutual community. Worrying about neighborhoods with high unemployment and being concerned about social barriers and the lack of social services helps convince people that they are a part of the larger nation.

Sixth, it is worth noting that the absence of a White House Iftar dinner this year and the persistence of the Trump Administration in seeking the so-called travel ban are unhelpful and, indeed, counterproductive. The travel ban would not have stopped the attackers from Manchester or London from coming here. It only bans citizens from six nations whose citizens have never staged a terrorist attack in the U.S. The travel ban was supposed to be temporary, to give the administration time to study the issue. The time they sought has come and gone. If they had used that time to do a study, it would be done by now. The absence of the travel ban during these last few months in no way prevented the administration from doing its study, if indeed it ever really was about a study. The growing perception in the U.S., right or wrong, that the administration is actually anti-Islamic puts us at greater danger.

Richard Clarke served as the national coordinator for counterterrorism on the White House National Security Council during Bill Clinton’s and George W. Bush’s administrations and was in the role before, during and after the Sept. 11, 2001, attacks. He is now an ABC News consultant.