The Nicaraguan government today effectively closed the investigation of Tuesday's assassination of newspaper editor Pedro Joaquin Chamorro, after one of the accused slayers said a Cuban-American doctor paid him $15,000 to arrange the crime.

Silvio Pena, one of four Nicaraguans arrested in connection with the slaying, testified that a Miami physician, Pedro Ramos, manager of a blood plasma exporting company here, assked him two months ago to hire the killer.

Ramos' company, Plasma Feresis, had been strongly attacked in recent months by Chamorro's newspaper, La Prensa, and the Cuban-born U. S. citizen had recently sued Chamorro for libel.

Last night in Miami, where he was visiting at the time of the assissination, Ramos said the accusation was "a monstrous lie" and a "smokescreen."

"I am the ideal victim to blame . . . I have never known Pena," Ramos said.

Nevertheless, well-placed sources said that Col. Aquiles Aranda, head of the National Guard's public relations office, said last night that the case was now closed.

But opposition groups, anxious to avoid abrupt shelving of the case, insisted that many questions remained unanswered, including whether anyone in the government gave the go-ahead to or offered protection for the assassin.

Although most government critics here do not think President Anastasio Somoza was directly involved in the assissination, the view among much of the population is that the government was in some way implicated. This belief prompted the widespread rioting and anti-government demonstrations that followed the killing of the government's most powerful opponent.

Plasma Feresis was the first Somoza-owned building to go up in flames and six of its U. S. employees fled to the U. S. embassy where they were permitted to spend Wednesday night. When the Plasma export business was formed in 1971, Somoza's private secretary was registered as holding 20 per cent of the shares, but Somoza has denied any connection with its Cuban-American operators other than renting them a building.

Whatever the outcome of the investigation, the immediate result of the slaying and its voilent aftermath, resulting in at least four deaths and $7 million worth of damage, is a further deterioriation of the political scene.

An unprecedented antigoverment campaign began four months ago with a series of guerrilla attacks. Although leftist guerrillas had staged isolated actions over the past 16 years, this time they received widespread support, largely because they said their goal was restoration of democracy rather than the achievement of socialism.