Police say Battisti was in a taxi attempting to cross into Bolivia and was found to have a large quantity of undeclared foreign currency.

In 1993 an Italian court sentenced him in absentia to life imprisonment for the murders, committed in the 1970s.

Brazil's ex-president Luiz Inacio Lula da Silva angered Italy when, in 2010, he refused to extradite Battisti.

Lula decided on his last day in office, 31 December 2010, to grant Battisti refugee status.

Brazil's O Globo news reports that Italy has asked Brazilian President Michel Temer to review Battisti's status.

As Italy has been exerting diplomatic pressure on Brazil over Battisti, the ex-militant may have been fleeing to avoid deportation.

Battisti has written successful police thrillers and has a Brazilian wife

O Globo reports that he is being questioned by police in Corumbá, a city near the border, about his foreign currency - reportedly $5,000 and €2,000 (£3,788 and £1,784 respectively).

Brazil caps at €2,700 the amount of undeclared foreign currency an individual can take out of the country, AFP news agency reports.

In the 1970s and early 1980s, Italy was plagued by political violence, including assassinations, by far-left and far-right groups.

Battisti admits he was a member of the radical Armed Proletarians for Communism (PAC) in the 1970s, but insists he never killed anyone.

He escaped from jail in Italy in 1981 while awaiting trial, then went on to became a successful writer of crime novels in France and Mexico, before escaping to Brazil in 2004.

From 2007 to 2011 he was imprisoned near the Brazilian capital Brasilia, before Lula ordered his release.