Shortly after last November's attacks on Paris by a Brussels-based Islamic State cell, a top US counter-terrorism official traveling in Europe wanted to visit Brussels to learn more about the investigation.
When the official tried to arrange meetings, however, his Belgian counterparts were not welcoming, according to US officials familiar with the events. The Belgians indicated it was a bad time to speak to foreign officials as they were too busy with the investigation, said the officials, who asked not to be identified.
Belgian officials declined to comment on the incident.
The brush-off was one small sign of mounting US frustration over Brussels' handling of its worsening Islamic militant threat.
Concern that the small European nation's security and intelligence officials are overwhelmed - and that its coordination with allies falls short - have again come to the fore following the Islamic State-claimed attacks on Tuesday that killed at least 31 people.
Several US officials say that security cooperation has been hampered by patchy intelligence-sharing by Brussels and wide differences in the willingness of different agencies to work with foreign countries, even close allies.
One US government source said that when American investigators try to contact Belgian agencies for information, they often struggle to find which agency or part of an agency might have relevant information.