A suspicious envelope was discovered at the U.S. embassy in Madrid and was placed under police control on Thursday – the sixth one found in recent days in the Spanish capital, including one that ignited at the Ukrainian embassy.

Over the past few days, explosive devices concealed in postal packages were sent to several places including Spain's Defense Ministry, an arms manufacturer and air base.

According to local broadcasters LaSexta, the package sent to the U.S. embassy on Thursday had "similar characteristics" to the previous five and was detonated by explosives specialists.

Earlier on Thursday, the country's interior ministry confirmed that Spanish Prime Minister Pedro Sanchez received a booby-trapped letter on November 24.

Speaking to journalists, Spain's deputy interior minister said that early indications suggested that all packages were sent from within Spain.

Rafael Perez, the junior minister responsible for security, said the homemade devices were sent in brown packages containing a flammable powder and tripwire that would generate "sudden flames" rather than an explosion.

"It appears that they were all sent from within the country, but we are basing this on early visual inspections without yet having an in-depth technical report," he said.

Perez said it did not yet appear necessary to convene the security committee that would evaluate stepping up Spain's terrorist threat level, but the Interior Ministry said in a statement that it had ordered police to strengthen security around public buildings and particularly check postal deliveries carefully.

Spain's High Court that specializes in terrorism has opened an investigation, Reuters reported, citing a judicial source.

(With input from Reuters)