Bahraini police on Wednesday questioned the head of the largest Shiite opposition group in the Sunni-ruled kingdom who was summoned after having met with a US official.
Ali Salman, a cleric who heads Al-Wefaq, was called in on Tuesday along with his political assistant, ex-MP Khalil Marzooq, by Bahrain's Public Security.
The interior ministry said they were questioned separately about meeting with visiting US Assistant Secretary of State for Democracy, Human Rights and Labour, Tom Malinowski.
It said the meeting at the US embassy violated a rule stipulating that contacts between political associations and foreign parties "should be coordinated with the foreign ministry and in the presence" of its representative.
The rule also stipulates that the justice ministry, which oversees political associations, should be informed of such meetings "at least three days in advance."
The interior ministry said the two opposition figures were allowed to leave after questioning, and that the investigation will be forwarded to the public prosecutor to decide on any future action.
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Al-Wefaq said on its Facebook page that Salman's lawyers were not allowed to attend and confirmed that the interrogation focused on the meetings with Malinowski.
The questioning came after Bahrain declared Malinowski, "unwelcome" after he met Salman and other opposition leaders, and demanded his immediate departure.
Bahrain's foreign ministry said Malinowski had met "with a particular party to the detriment of other interlocutors," describing his action as an "interference in its internal affairs."
Malinowski was the Washington director for Human Rights Watch, a vocal critic of Manama's crackdown on protests, until April when he became assistant secretary of state.
Shiite-led protests erupted in Bahrain -- home base of the US Fifth Fleet -- in February 2011, taking their cue from uprisings elsewhere in the Arab world and demanding democratic reforms in the absolute monarchy.
Security forces boosted by Saudi-led troops crushed the protests a month later, but smaller demonstrations frequently take place in Shiite villages, triggering clashes with police.
The foreign ministry said relations between Manama and Washington would not be affected by the "unfortunate acts" of Malinowski.
Bahrain is a strategic archipelago just across the Gulf from Iran, and Washington is a long-standing ally of the ruling Al-Khalifa dynasty.