(CNN) -- About 20 police officers entered the tightly guarded home of Myanmar opposition leader Aung San Suu Kyi on Thursday, a day after authorities detained an American swimming away from the property across a lake.
Pro-democracy supporters hold a portrait of Aung San Suu Kyi in New Delhi, India, last November.
"We heard about it this morning. We don't know the reason yet," said Nyan Win, spokesman for Suu Kyi's National League for Democracy. "We think they are investigating the event of one U.S. citizen who went to the house using the lake."
The U.S. Embassy in Yangon confirmed the arrest of the American man, saying it received an official notification from the country's foreign affairs ministry. It, however, has not had contact with the arrested man, said spokesman Richard May.
Suu Kyi has been under house arrest for 13 of the past 19 years and is rarely allowed visitors, except her doctor. Police block the entrance to her lakeside home in Yangon, formerly known as Rangoon. And swimming in the lake is forbidden.
The American, identified by local media as John William Yeattaw, apparently swam across Inya Lake on Sunday and sneaked into the house. He was arrested while swimming back on Wednesday, the Myanma Ahlin newspaper said.
The newspaper is affiliated with the country's military junta.
The newspaper account did not mention whether the man met with Suu Kyi. Nyan Win, the party spokesman, said he hadn't.
"He's not met with the Lady," he said, using the honorific for Suu Kyi. "He was hiding in the house."
"The Lady is concerned about security in the compound. This event shows there is no security there."
Media accounts of the incident said the man confessed that he was visiting Yangon on a tourist visa and was staying at a hotel when he swam through the lake using a 5-liter water bottle, presumably to use as a float.
Authorities found an American passport, a black backpack, a flashlight, a pair of folding pliers, a camera and money on him, the newspaper said.
Suu Kyi, 63, was first detained in 1989 and authorities have extended her detention regularly. She won the Nobel Peace Prize in 1991.
The latest extension expires later this month, but it is unlikely the junta will loosen its restrictions on her movement, her party said.
Myanmar, also known as Burma, has been under military rule since 1962.
Suu Kyi rose to global prominence during protests in the southeast Asian country in 1988. She was placed under house arrest before her party won the 1990 general elections, which the ruling military junta did not recognize.
Myanmar's government has scheduled elections for next year that it says will lead the nation toward democracy. Human-rights organizations have said the vote will merely extend military rule in the nation.
CNN's Ben Brumfield contributed to this report.