WASHINGTON, Nov. 1 (Xinhua) -- U.S. President Donald Trump on Wednesday blamed a 20-year-old visa program for opening doors to the New York City attacker who killed eight people Tuesday by mowing them down in a hired truck, saying the Democrat senator who backed the program should bear responsibility.

"The terrorist came into our country through what is called the 'diversity visa lottery program', a Chuck Schumer beauty," Trump said in a morning tweet, pointing fingers at the Senate Minority Leader.

"No more Democrat Lottery Systems, We must get MUCH tougher (and smarter)," he said.

The United Statesstarted the visa lottery program in the 1990s, allocating 50,000 visas each year to people from countries that have relatively few immigrants in the United States.

Citizens from eligible countries who hold a high school diploma or have two years in a professional occupation can put up their names in a pool, from which a computer chooses the successful applicants.

Sayfullo Saipov, identified as the attacker, was said to have entered the United States from Uzbekistan on a visa "won" through the program.

Schumer, then a congressman representing the state of New York, played a key role in coming up with the program, which was ultimately passed by Congress as part of larger immigration policy package.

"I guess it's not too soon to politicize a tragedy," Schumer said on Twitter in response to Trump's tweet.

Jeff Flake, a Republican senator who recently had a fallout with Trump, defended Schumer on Twitter, saying the senior Democrat was part of an initiative in 2013 to scrap the diversity visa program.

Aside from taking a swipe at his political opponents, Trump also sought to promote his version of immigration reform, which two Republican senators rolled out in August, featuring a merit-based immigration policy.

"I want merit based," Trump said in his tweets, "we are fighting hard for Merit Based immigration."

The merit-based immigration policy, named Reforming American Immigration for Strong Employment, would curb low-level immigration by examining the age, level of education, salary and English proficiency of visa applicants.