This story has been updated

TO HELP friends and family members find one another amid the crowds expected at this year’s Boston marathon, the organisers set up a series of meeting points in the streets around the finish line. The bright yellow and orange signs are still there, but most of the planned reunions were thwarted—and a few will never now take place—thanks to the two bombs that blew up on April 15th near the end of the course in the closing stages of the race.

The explosions, separated by 600 feet (180 metres) and 12 seconds, killed three people: an eight-year-old boy and two young women. Another 180 or so were injured. At least 13 people lost limbs. That makes the attack the most deadly act of terror in America since September 11th 2001, apart from mass shootings. To add to the alarm, the authorities intercepted two letters, one addressed to Barack Obama and another to Roger Wicker, a senator, which appeared to contain ricin, a lethal poison. A fire followed by an explosion at a fertiliser factory in west Texas on the night of April 17th, which killed at least five people, increased the sense of apocalypse. But as The Economist went to press there was still no indication that this was anything but a horrific accident.

Shortly after the Boston bombing Mr Obama vowed that “any responsible individuals, any responsible groups will feel the full weight of justice.” The FBI, sifting through the mass of video footage and photographs provided by broadcasters, security cameras and bystanders, homed in on shots of two individuals who officials believe may be responsible for the bombs. But their whereabouts, identity and motives remained a mystery. An aggrieved constituent of Mr Wicker was arrested on April 17th, suspected of sending the poisoned letters; but he has not been tied to the bombings.

Boston is now flooded with police, soldiers and other security forces, both to ward off further attacks and to help with the investigation. State troopers stand watch outside metro stations; whole squadrons of national guardsmen march back and forth on Boston Common, a park a few blocks from the site of the bombings. Barricades festooned with flowers and notes of condolence block access to the immediate area of the blasts, as investigators pore over it in the hunt for evidence.

Fragments retrieved from the scene suggest that the bombs were constructed from six-litre pressure-cookers packed with gunpowder, a simple detonator and shrapnel in the form of nails and ball-bearings, intended to cause horrific injuries.

The simple but effective design of the bombs, which may have cost as little as $100 to make, is similar to countless improvised explosive devices (IEDs) that have inflicted casualties on civilians and Western troops in Afghanistan and Iraq. Instructions for building such bombs can be easily found on the internet. The bombs’ lack of sophistication suggests they were either the work of a lone individual or a small group with limited resources.

In 2010 an online magazine, Inspire, published by al-Qaeda in the Arabian Peninsula, ran an article entitled “How to make a bomb in the kitchen of your mom”. Inspire, aimed at aspiring young jihadists in America and Britain, was edited by Anwar al-Awlaki and Samir Khan. Both were American citizens who were killed by the same drone strike in Yemen in 2011.

However, the fact that pressure-cooker bombs are popular with al-Qaeda operatives is far from conclusive evidence that the Boston bomber had jihadist aims or connections. America has suffered several terrorist attacks by native-born extremists with domestic grievances, such as the IED set off near the Olympic stadium in Atlanta in 1996, or the bombing of a federal office building in Oklahoma City in 1995.

The bombings in Boston took place as locals celebrated (four days early) Patriots’ Day, the anniversary of the first skirmish in America’s war for independence. The date could be significant to the sort of radicals who believe the federal government is bent on the destruction of their liberties and the confiscation of their guns, especially as it coincides with the anniversaries of two violent episodes involving like-minded people: the end of the siege of the compound of a heavily armed cult in Waco, Texas, on April 19th 1993, and the Oklahoma City bombing exactly two years later. April 15th is also the day by which federal income-tax returns must be filed, another flashpoint for many.