At least a week ago, U.S. immigration and Customs agents received identities of 12 alleged terrorists who make up the suspected Libyan "hit squads" that have caused such a furor in Washington recently.

Immigration and Naturalization Service headquarters in Washington sent a seven-page memo to its major border-crossing and airport offices last Sunday containing the names, ages, aliases and sketches of five members of one team believed led by an international terrorist known as Carlos. It said "targets of this team are the president and other high government officials."

The memo, which was labeled "sensitive, for official use only," was described in Thursday's San Diego Union. Syndicated columnist Jack Anderson released the sketches the same day.

There was some confusion about the sensitivity of the information because it was found posted on the lunchroom bulletin board at the Immigration and Naturalization Service border-crossing station at San Ysidro, Calif., Tuesday afternoon.

But sources said that the information was sent to the field because it was, and is, the best information the intelligence community has about the identity of members of the alleged assassination squads.

One source cautioned, however, that the identities and sketches are based mostly on the recollection of a key informer whose credibility has not been determined. The Union article quoted the INS memo as saying that information "from a reliable source indicates that a . . . hit team headed . . . by Carlos may attempt illegal entry into the United States via Mexico during early December, 1981."

Carlos, 32, whose full name is Carlos Ilich Ramirez Sanchez, was reported to be traveling under an assumed name, "is skilled at changing appearance and identity" and should be considered "extremely dangerous," the memo said.

The Venezuelan-born Carlos is believed to have planned the 1972 Munich Olympics massacre of Israeli athletes and coaches, the 1975 kidnaping of Organization of Petroleum Exporting Countries oil ministers in Vienna and a 1976 hijacking of a French jetliner to Uganda that ended with the Israeli commando raid at Entebbe.

According to the Union article, the memo said that the other members of Carlos' team included three Syrians and three Libyans. A second team was said to include two Iranians, a Palestinian, a Lebanese and an East German.

Libyan leader Muammar Qaddafi has denied reports that he ordered an assassination team into the United States to go after President Reagan and other top U.S. officials. But American intelligence officials have said they have been forced to take the information seriously.

Thursday's disclosures on the names and sketches of the suspected terrorists is the most complete information yet about the alleged plot. But American officials acknowledge that they are puzzled over how anyone could obtain the kind of detailed information the INS received from the intelligence community.

Secretary of State Alexander M. Haig Jr. said yesterday that the assassination-team intelligence came from a "host of sophisticated intelligence-collection capabilities." Others have said much of it came from the one informer.

The Los Angeles Times reported today that Israeli intelligence was the major source of some of the most dramatic published reports about the suspected plot. The paper said Israel's intent in passing on information to the news media about the affair, including assertions that Carlos heads one of the teams and that the suspected terrorists were in Mexico or Canada, was not immediately clear.

But it quoted sources as saying that Israel may be trying to build American public support for a strike against Qaddafi.

An Israeli Embassy spokesman had no comment on the report of Israeli involvement, the Times said.