MADRID—The arsenal is a terrorist’s dream: 150 live hand grenades, 44 rocket propelled grenades, 1,450 9mm cartridges, 18 tear gas grenades, scores of triggers and detonators of various kinds, 102 explosive charges, and 264 blocks of plastic explosive. Such is the inventory of deadly materiel that was stolen from a military installation in Portugal on June 28 and is still missing.

Then, two days after that robbery, a van loaded with nitroglycerin was robbed in Barcelona, Spain. Those explosives have not been recovered either.

European authorities are worried, to say the least. These are not the unstable homemade munitions used in many recent terrorist attacks, they are military-grade. But precisely who took them, and for whom, remains a mystery.

In the June 28 incident, more than a dozen thieves stormed the military armory of Tancos, located about 80 miles from Lisbon. The Portuguese government, which immediately notified the Ministry of Interior of the neighboring country, Spain, has since convened various national security meetings and launched an investigation the details of which remain carefully guarded secrets.

On Tuesday, July 11, Portuguese Prime Minister António Costa met with senior military officials to analyze the security of the country’s military installations and gather data on what had happened in Tancos. In the subsequent press conference he insisted he felt “calm” based on such information as the military had provided about the theft.

The commander of the Portuguese armed forces, for his part, tried to send a message to the thieves when he spoke at the press conference: “The rocket launchers probably can’t be used effectively, because they were slated to be destroyed. They do not have the dangerous potential that those who took them think they have.”

Apparently reasoning along those lines, Portugal decided not to raise the anti-terror alert, but elsewhere in Europe, and in particular in Spain and France, the closest countries, news of the theft was greeted with consternation.

Portugal’s hypothesis, despite official efforts to play things down, is that the robbery actually is linked to jihad. Meanwhile police sources in Spain believe that a criminal non-terrorist organization was behind the theft. But that does not really diminish the threat: Such criminal organizations sell the weapons to the terrorists.

The controversy in Portugal has caused a political tsunami, because the theft has brought to light the lamentable security measures of the Tancos base: The video surveillance system was damaged five years ago and had not been repaired, the motion sensors do not work, the wire fencing is vulnerable to a good pair of scissors, and the 25 watchtowers are in such bad shape soldiers don’t dare to climb them.

The thieves may have gotten their inspiration from the official government bulletin itself, Diário da República, which on June 19 called for a €316,000 ($368,000) tender to repair the north, east, and south of the fence of the military base of Tancos.

In Barcelona, on the other side of the Iberian peninsula, dozens of containers of nitroglycerin were taken from a Toyota Proace belonging to the company Fike Solutions—which carries out controlled explosions. The robbery occurred in the company’s parking lot. Although the police later found the van abandoned without the explosives, there is no official information about the robbery.

All this has happened as Spain is in the midst of a continuous anti-jihadi operation. Every day police forces announce the arrests of terrorists, although most of them are being picked up for proselytizing, not acting.

Spanish interior ministry sources say that the jihadis detained recently in Malaga, in southern Spain, planned to buy weapons and bulletproof vests in the country. And on July 12 the National Police detained in L’Hospitalet de Llobregat, Barcelona, a Spanish citizen of Palestinian origin accused of “incitement to hatred for religious or xenophobic reasons and for his alleged participation in a terrorist organization.”

The detainee came from a Palestinian refugee camp in Lebanon and had expressed “his hatred of Israel, the Shiites, Spain, and the entire West.” According to police, he was in “a high degree of Islamist radicalization” and was a faithful follower of Salafist cleric Khaled Al-Rashed, in prison in Saudi Arabia after appealing to Muslims around the world to join the jihad in Iraq.

In 2004, Spain was the target of a devastating attack with multiple bombs placed on commuter trains and detonated as they arrived at the Atocha station in central Madrid: 192 people were killed and 2,000 injured. The explosives used: stolen dynamite.

In June 2015, after years of relative quiet, Spain raised the level of its anti-terror alert for three weeks, and since then, in operations abroad as well as inside the country, Spanish security services have detained 185 jihadis. Altogether since the beginning of 2015 the detainees for Islamist terrorism in Spain have amounted to 230. Citizen collaboration in a program called “Stop Radicalism” helped launch the investigations that led to some of these detentions.

A significant number of the jihadis detained in the last year were already deeply into the planning phase for attacks on Spanish soil. An example is the man detained in Madrid last June 21, a 33-year-old Moroccan. According to the court order, police found a virtual library of the Islamic State in his home, evidence of his contacts with jihadi fighters through social networks, and “a note of marked apocalyptic tone” left at his workplace on June 13—after the end of his labor contract. It read, “Dominio Jihad today returns to Paradise.”

Worse still is that the detainee, monitored by the secret services, had begun to reconnoiter “on the spot” landmarks in the center of Madrid. Police believe he was in the first phase of a planned terrorist action in the Spanish capital.

In short: Active jihadis are at large in Spain and in Europe. And now the explosives are, too.