MADRID — When a European arrest warrant was issued recently for Brett and Naghemeh King, who took their cancer-stricken child out of a Hampshire hospital in the south of England without permission, the Spanish police did what has become increasingly common in the search for missing or wanted people: They posted an alert on Twitter.

Within three minutes, the police received a Twitter message saying that the British family’s minivan had stopped for gasoline the night before on a highway in Eskoriatza, in northern Spain, and was headed south. In coordination with British authorities, the police centered their search on Spain’s touristy southern coast. Two hours later, they received a call from a hotel receptionist near Málaga, who had seen the police alert and checked the license plate number on the minivan. The parents were then detained.

The department’s Twitter account, @policia, has become the most popular maintained by a law enforcement agency — just ahead of the FBI’s — with a million followers in a country of 47 million people.

Police forces in Latin America and elsewhere have visited Spain recently to replicate its police department’s Internet strategy. This month, the New York Police Department announced that commanding officers would take a course on using Twitter and how to release better information online about investigations.

As Twitter and other social media become more prevalent, however, some police forces have had problems related to privacy laws and the posting of inappropriate messages by individual officers. In New York, the Twitter training is being conducted after a police officer posted a personal message in July that made light of the death of a woman who had fallen onto the subway tracks while using her iPad.

In the case of Ashya King, the boy with brain cancer, the police in Hampshire have been under pressure because of their handling of the case, including postings on the Internet. The Hampshire police “lookout” alert, accompanied by mug shots, was criticized as portraying the parents as hardened criminals, rather than people seeking alternative medical treatment for their son.

On Tuesday, the parents were released from custody in Spain after British prosecutors dropped the case.

Under Britain’s data protection act, the police have wide latitude to use and spread information. Still, the British police have investigated as many as 828 cases in which officers have been accused of abusing social media, including posting racist and threatening comments, according to documents obtained last month by The Associated Press.