A long RP tachycardia: Where is the culprit?

A 28-year-old woman presented with nearly incessant palpitations. The symptom had first occurred when she was 6 years old and had increased in frequency over the past several months. Her baseline electrocardiogram (ECG) was normal. Echocardiography showed normal atrial and ventricular sizes, with left ventricular ejection fraction of 67%. Family history was negative. Figure 1 shows a 12-lead ECG recorded during palpitations. What were the possible mechanisms of the tachycardia?